Scared Shitless: Loss of Bowel Control?

I am not aware of consciously controlling my bowel movements. When I feel the call of nature I can resist for awhile before I must actually go and defecate, for example, when you have to wait on a line to get your opportunity to use the bathroom. I thought that the feeling of having to empty your bowels was an involuntary response that comes whenever it comes. So how can a sudden fright cause someone, a young healthy person (ex: a soldier in battle) to **** themself - unless they already had to go to the bathroom in the first place?