There's a series of reasons.
First, my GI has always been troublesome. Ever since childhood. It didn't become pathologically troublesome until my colon cancer arrived, but I've always carried my stress in my colon. Where some people get back spasms or migraines or sick at their stomach as a stress response, I always got gas or diarrhea as a stress response. I'm talking weapons-grade gas here, as some of my friends can unfortunately attest.
Second, my initial cancer presentation was primary colon cancer. This may or may not be correlated with the stress issue, but that's unprovable at this point. In any case, 22 cm of my sigmoid colon was removed in May of 2008. That's pretty much the whole descending colon, which is the part that serves as the 'brakes' on elimination processes, and provides storage for and water absorption from feces. In other words, even under ideal conditions, my poop comes out sooner, faster and wetter than it used to.
Third, my gall bladder was removed co-incidental to my first liver resection in 2010. There wasn't actually anything wrong with it, but the tissue to which it attached was being resected, so the gall bladder came along for the ride. The gall bladder's function is to control the availability of bile, which is essential in metabolizing dietary fat. My body still produces bile, but the bile is no longer regulated, so my fat metabolism is functionally random. This means the same food that is just fine one day can send me sprinting for the toilet the next day.
Fourth, the ravages of chemotherapy. Speaking very crudely, chemo works by targeting fast growing cells. Hence the classic cancer patient side effects of hair loss, nausea and immune system dysfunction — hair cells, stomach cells and bone marrow cells are very fast growing. But it's not just stomach. The whole GI is like that. Basically, chemo hoses out the GI, and in my personal case, the effects are much stronger in the lower GI than the upper. Those effects can cover the whole gamut of imaginable GI problems, and in my case normally follow a fairly specific sequence.
So, even leaving out chemo, I have a pretty iffy digestive system, specifically my lower GI processes. When I'm on chemo or in chemo recovery, that gets amped up to an alarming degree.
Hence all the GI issues.