It's rare but can happen where long distance running causes ischemic colitis which is where on a long run enough blood is diverted from the large intestine that it damages the intestine long term. It isn't surprising to me that there's higher likelihood of colon cancer given this. It seems like repeated bouts of lower blood for the intestine on long runs has a cumulative impact and damages the colon even if it doesn't cause ischemic colitis.

This theory has been put forward, but it's important to point out that there is no real evidence yet. An alternative theory is diet, which is also the leading theory for increasing incidences in non-athletes. Highly processed, calorie dense foods have been on the watchlist for a while, and ultra endurance athletes have a special need for these to satisfy their caloric requirements. It could also be a combination of these factors or something else that was missed entirely so far.