The reasons why it's happened to guys I've been with in the last three-or-so years:
1) They were in a rush: there's a sequence here that's got to be followed. Douche, expel, wash, wait, repeat. The sequence takes at least an hour, sometimes longer.
2) They were not thorough: they skipped a step, usually the last two.
3) They were trying to be spontaneous: anal sex is not a spontaneous act. It requires preparation.
Each partner has the responsibility to bring all he or she can to each encounter. I have pretty significant digestive issues of my own: both the meds and HIV itself wreak havoc on someone's gut. If I'm not certain that I can enjoy an extended period of time focused on pleasure rather than what's going on in my colon, I skip it. I expect no less from a bottom.
Of course accidents can happen, and when they do I'm gracious but firm. I won't go near his hole again that night, and will think hard and long before agreeing to a follow-up. It's a sea/fish thing.