And that reminds me of this thing I read

Wherein slightly related one of the older students at The Child's ballet school has such a need to void the contents of her stomach before a performance that puke buckets are placed on each side of the stage


Slate asks Can You Be Scared Enough To Pee Your Pants? Simple answer is yes and Brian Palmer does provide a reason. However, for a more colorful explanation, let me direct you towards Neal Stephenson's Reamde:

No one could look at Peter, who had become a nearly unbearable sight on grounds of posture alone: shoulders drawn together, body trembling, back of neck brilliant red. Sokolov was favorably impressed by the fact that he had not yet shit his pants. Men always made crude jokes about people pissing their pants with fear, but in Sokolov's experience shitting the pants was more common if it was a straighforward matter of extreme emotional distress. Pants pissing was completely unproductive and suggested a total breakdown of elemental control. Pants shitting, on the other hand, voided the bowels and thereby made blood available to the brain and the large muscle groups that otherwise would have gone to the lower-priority activity of digestion. Sokolov could have forgiven Peter for shitting his pants, but if pissed his pants, then it really would have been necessary to get rid of him.