I don't have IBS that I know of, but on the morning before a ride, I am always in and out of the bathroom several times. I drink coffee, which does trigger it, but even before coffee, it seems to be triggered by just thinking about riding! I don't have this issue on work days or non-riding weekend days, other than rarely; but it's like clockwork on riding days. Because of that, I do think it's in large part psychological in my case. It's actually a good thing in that once I am all "cleaned out" I never have to think about my bowels during a ride.

DH has this same issue on riding days...we are like two peas in a pod. Fortunately, we rarely do group rides/events where there is a set start time, so we can just trade turns in the bathroom until we're both completely done. Sometimes that means leaving later for a ride than we'd planned, but so be it.