Serious question, I hope, though it might not sound so. Someone 'crapping themselves' through fear (or maybe any other strong emotion) is quite real. And even if it doesn't go to that full extent, the bowel contents often seem go through an instant liquidiser when things turn grim. It is known why (or how) what obviously starts as a state of mind turns into a physical reaction? For example does the body start over-producing some chemical that causes the effect on the digestive system?