This is a job for Dr. Superfly_House.

This is indeed a medical mystery. When you're nervous (you can't control this) the sympathetic nervous system kicks in hence the quavering voice. However, the sympathetic nervous system has the opposite effects on your bowel movements. The sympathetic system is responsible for the "fight or flight response" which prepares your body to be able to survive a dangerous encounter, so:
wiki wrote:
the sympathetic nervous system can accelerate heart rate; widen bronchial passages; decrease motility (movement) of the large intestine; constrict blood vessels; increase peristalsis in the esophagus; cause pupil dilation, piloerection (goose bumps) and perspiration (sweating); and raise blood pressure
So basically, when you're nervous you shouldn't be feeling the need to shit. Shitting is the parasympathetic nervous system which is what kicks in when you are calm and relaxed:
wiki wrote:
the "rest and digest system", the parasympathetic system conserves energy as it slows the heart rate, increases intestinal and gland activity, and relaxes sphincter muscles in the gastrointestinal tract.
What this means is that you either have something like chronic bowel syndrome (not dangerous...very common) where shitting can be triggered by stress, or your body just reacts differently to jitters because the nervous system is not absolute in everybody. Of course, since i'm playing Dr. Superfly_House I must also include the differential diagnosis of Sarcoidosis