At least in my own anecdata of IBS, caffeine alone does not have a laxative-like effect for me. I have noticed some but not all strongly bitter foods do, and heard a theory once that some flavoring compound that tastes bitter may be subtly enough like gall passes through the stomach to confuse the gut into thinking there may be a gall leak and to flush the system faster. That certainly sounds right to my anecdata, but also part of why I think I crave bitter foods sometimes (as an IBS-C to IBS-D switch/trigger).