I think the way racehorses are managed is the biggest factor. 90+% of them have gastric ulcers. I can only imagine what the average racehorse colon looks like given the widespread use of bute and other "acceptable" management practices.
Seattle Slew had it early in his 4-year-old season, before the drug restrictions were relaxed/abolished. But then again, he was reputedly a pretty hot dude and difficult in his stall. He may very well have had some nastiness going on in his colon.
Well it's hereditary in humans, why not horses? My OTTB has Inflammatory Bowel Disease, the vets believe it might be hereditary as well. It is believed to be caused by poor diet, stress, continual use of antibiotics, viral and bacterial infections, and parasites. But nobody has found the direct cause, these are just what vets believe cause these kinds of diseases. Because of these triggers, TBs are prime candidates for gastro-intestinal disorders.