Good question.
It depends. If a bottom cleans out too deeply--like up into the sigmoid colon--it can bring more poop down. I know this from going too far preparing to bottom myself.
Sometimes there's a fine line between cleaning out only my rectum and going further. If I get into the sigmoid cleaning out can take a lot more time than rectum only. Depends on what's in the pipeline. I didn't start bottoming until I was 40. It's something I learned from experience. I'm guessing most who bottom do.
I recommend cleaning out the rectum only. Again this is learned skill.
I top way more than I bottom and I'm long enough to get through the "second hole" into the sigmoid when I top. Sometimes I get into poop. It can be messy (what mudslide isn't?) and it may ruin the moment but normally it's just a delay while a bottom goes and cleans out again before we resume. While that's happening I'm exchanging a muddy condom for a clean one.
It's a sewage outlet. Sometimes it takes more than once to clean it out sufficiently.
Another factor is that a lot of bottoms play with big toys that go deep and already know how deeply they need to clean to avoid mudslides. Again it's a matter of experience.
Blame? Yeah I've been blamed for mudslides but guys I've topped have apologized as well. To me it's not about a blame game. Question is do we want to clean up and continue or abandon the enterprise and maybe try some other time.