Why or when does this not happen?
Prior to getting to near the end of your digestive tract, the ascorbic molecules or their two electrons may be used by the body. This could happen anywhere between your mouth and your colon. It may be "called out along the way," such as going 'through' or 'into' tissues like blood or lymph macrophages and more. Ascorbic is used for so many things in the body, affecting so many organs, systems, hormones and fluids that even after nearly a century of science it is not all known.
Suffice to say that your body knows how to use it, if only you can get it to the parts of your body that need it. You can literally die via "acute localized scurvy" even when your bloodstream has a decent amount of C in it. What the tissues have is a separate measure, and different tissues hold different amounts, and even the plasma can hold a different amount than a whole-blood measure.
Some illnesses that are very virulent kill people not through the illness, but through the localized-tissue acute scurvy the illness creates. Some illnesses that are not very virulent, still kill people that way, because doctors are indoctrinated by an industry that wishes to evade unpatentable substances which preserve health, let alone cure illness or injury, and they simply do not know that the patient may have acute localized-scurvy (which is deadly) as part of (or a result of) their condition.
How much of the ascorbic is "already used up" by the time it hits your lower digestive tract is going to depend on what's going on in your body. If you are fighting off the flu, or you've just had a serious body-shock (psychological or physical), your body is going to use it in every way it can when you ingest it.
Your body may or may not be able to get much to a given set of tissues, depending on how much you take. Depending on your 'extra need,' there may be very little (if any) ascorbic left by the time your originally ingested dose gets to (or would have gotten to) the large intestine. Cathcart's bowel tolerance experiments (see figure below) suggest the "possibility" that perhaps the ascorbic is not "forced" fully into all body tissues until a certain critical amount is present.
Now let's look at why it does happen.
Ascorbic has an "osmotic effect" in your intestines. It draws water inward. The more ascorbic (or ascorbate) you have in your intestines, the more water it pulls in. The longer it is sitting in your intestines, the more water it pulls in. So, the larger the amount of ascorbic that gets to your intestines, and the longer it's there, the more water your intestines are going to have in them, making an internal-enema condition.
How much ascorbic gets into your intestines will depend on how much you take, but also how much your body 'uses' first. How long it stays in your intestines may be affected by your gastrointestinal system and by what else you might ingest it with. A lot of food might slow it down, on one hand; a lot of bad stuff might use some of it up on the spot.
So the ascorbic is not pulling in water because you have "too much" of it. (That is a common misunderstanding.) Too much being a relative term of course -- 'too much to not escape bowel flush' is the only way that could be fairly used. Ascorbic was originally created inside the body naturally, so mankind did not come about with the body expecting to absorb those quantities through the bowel: it made ascorbic locally (initially in the kidneys, then later in the larger liver). So we are actually trying to "force" it into ourselves in a way the body is only somewhat adapted to; it is adapted to absorbing it from food, but in vastly smaller amounts.
Osmotic diarrhea is caused by the presence of unabsorbable or poorly absorbable solute in the alimentary canal. ... Osmotic diarrhea may occur with a normal dietary intake, if digestion and absorption are impaired, or it may occur from ingestion of solutes for which there is no absorptive mechanism. ...the net effect of osmotic diarrhea is water and potassium depletion.-- M. Ellert, Nutrient Absorption, 1998 http://www.siumed.edu/mrc/research/nutrient/gi42sg.html
Ascorbic is one of the most non-toxic substances you can ingest, even in gigantic doses. The reason it creates "water in the bowel" is simply because that is its nature and it is there. That is not bad (actually ascorbic can be great for the digestive tract in a variety of ways), although it may be uncomfortable and unpleasant. There may be times when a person would want to suffer the bowel flush (even in a major way) on purpose and keep taking ascorbic despite that (it may be that getting the ascorbic or its electrons to tissues in high quantity as fast and continuously as possible is a dire situation).
How to reduce the bowel-flush effect
Let us say that you really want to take 12 grams of ascorbic a day. Alas, perhaps on average your body gets the 'flush effect' after about 3 grams. There are a couple of things you can do.
1. Divide the dosage. Remember the issue is "ascorbic sitting around in your intestines pulling in water." So, the less ascorbic sitting around in your intestines at any one time, the less water. Rather than taking a daily dose of 12g, take four doses of 3g, or even depending on how desperate you are to use it rather than lose it, take it in 12 doses of 1g. Your body is much more likely to use up "more of the dose" this way, reducing how much ascorbic is left to sit around in your intestines pulling in water.
2. Buffer the acid. I don't have a ref for this but allegedly the buffered forms of ascorbic (which become an ascorbate) may absorb a slightly larger percentage in the intestinal tract. (By slightly larger I believe we're talking maybe 5-10% or so.) The more you absorb, the less is sitting there pulling in water, so this seems like a good thing.
3. Emulsify the acid. I don't think the science has been done or published on this to know what is true or not, but there is a 'theory' that possibly, emulsifying the acid with a lipid (such as lecithin) may cause some of it to be absorbed by the body in other ways (perhaps through the peyer's patches in the intestines). This may not be true; I don't know. One theory about home-brew lipo-C is that it may be less 'liposomally encapsulated' than everyone hopes, but it clearly does allow a larger intake without bowel flush, so one theory is that a simple emulsification from the process may relate to the results.
4. Lipsomally encapsulate the acid. This part does have science on it. Lipospheres less than 200 nanometers are said to be sent by the liver out to the body for direct use (which should have about zero bowel effect); spheres larger than that, are dis-assembled at the point of the liver (which may still have less bowel effect than not being encapsulated at all) before the materials are 'handled.' Note that commercial lipsopheric encapsulations may be significantly different in encapsulation percentage and/or liposome size, and hence results, than 'home-brew' "lipo-C" (of which I am a fan).
When one experiments with how much they can take (in what dosages and at what frequencies), compared to how much it helps them (from general well-being, to dealing with symptoms or infections), this process of experiment to come to a decision on what the person feels is a good middle-ground between 'relief of symptoms' versus 'inconvenience of bowel-result' is called "titrating."
Why this is actually kind of interesting
The way the bowel tolerance changes actually tells us a lot about what is going on in the body. If you can absorb 200 grams of ascorbic in a day without a flush effect, you probably have a fever; you definitely have a problem which it's working hard to take care of.
Dr. Robert Cathcart did experiments with thousands of patients over nearly a decade, with bowel-tolerance titrating, to evaluate how much peoples' bodies would take before reaching bowel-tolerance when they had a given condition, from 'ordinary seemingly healthy' to 'viral pneumonia.'
One of the things he discovered is that the cessation of primary symptoms -- in other words, the point where the ascorbic appeared to reach "critical mass" to outweigh the assault it was fighting -- happened just before bowel tolerance. So for the bloodstream, bowel-tolerance is the point where the ascorbic cannot fight off whatever-else is going on any faster; more ascorbic than that, taken orally, is just going to flush. (This may however be different for isolated areas of tissues.)
It also provides some 'average' idea of how much daily ascorbic is going to be necessary to take care of various conditions, and how much one is going to have to take before any sign of improvement is likely to show.
Click on the image to see the full size.
|Figure-Table-Abstract-Summary from Cathcart Robert F.|
Vitamin C, titrating to bowel tolerance, anascoremia, and acute induced scurvy.
Med Hypotheses. 1981;7:1359–1376. PMID: 7321921
Above is a composite image I made summarizing the table, figure, abstract of his 1981 paper on this. Please see the full white paper which is online here:
Much of what little we know (which may or may not be reflected in this post) is thanks to docs like Klenner and Cathcart who did a massive amount of work over many years in experimenting and documenting the myriad uses and details of ascorbic for health.
 Yeah I know that probably, having 'bioflavenoids in context' is a better way of intaking C, however officially and in most practice, ascorbic acid, with or without chelates or bioflavenoids, is C.
 An effect that causes the intestines to pull water back into the colon and hold it in the intestines, softening stools so they're easier to pass.
 To titrate: ascertain the amount of a constituent in (a solution) by measuring the volume of a known concentration of reagent required to complete a reaction.
Constant caveat: I am not a scientist. For "real science" you should read articles by people with academic credentials, of which I have none.