When running fast or long distance and you crap your pants.
Tim ate a power gel that messed up his stomach and he got the runner's trots.
When running long distances, you all of a sudden feel an extreme urge to poop and if you don't get to a toilet fast, you're probably going to poop your pants...bad! Runner's trots usually are a precursor to some nasty diarrhea.
Only a mile into the marathon, Jan got runner's trots and had to high-tail it to the nearest port-o-potty.