Runners often experience a condition known as "runner's diarrhea," which is when the person engaging in the activity experiences the sudden need to defecate during or after a good run.
The condition occurs not only in hobbyists who are looking to shed a few pounds by jogging around the block, but also in professional runners, like those who participate in marathons.
One of the possible causes behind the sudden urge to poop is the rise in body temperature from these activities, which can melt things down and get them moving. This is muchmore likely to happen when you also have the forces of your body pounding the ground with each footstep in the case of running.
Since there's hardly a way to prevent this from happening to those who experience it, it can help to stay hydrated, as this helps regulate body temperature. Trying to go #2 before running can also help prevent an embarrassing situation from happening during the run.