So I've always wondered what about running makes you have to go to the bathroom? Why is it that it seems to accelerate the downward progress of any ... you know ... Or maybe I'm the only one this happens to?
You're not the only one this happens to, exercise is one of the best natural laxatives.
What I found interesting is that any physical activity can help prevent constipation, because it isn't just the physical rebounding of the gut from running which loosens things up, the colon also gets stimulated so it can both absorb the nutrients you'll need while exercising and expel waste more efficiently. So, any exercise will actually have a laxative effect.
It may be a combination of running and your diet. Generally, you should stay away from a high fiber and high fat diet 24 hours before a long run. I also recommend holding back on as much fiber and fat as you can until you complete a regular run for the day.
"Runner's trots" or an increase in the need to hit the port-a-john during or after a run is most often attributed to a general decrease in blood flow to the intestines, altered hormones and absorption levels, and “jostling” of the gut (think about what happens to a ketchup bottle when you turn it upside down and smack it).
Here's a link to an short article that goes a bit more in depth.