Bikinimom, it is actually an adrenaline response. We send all our blood supply to our muscles to work (aside from supporting basic life function) so we have to give up digestion for a while. So as the blood supply is lessened for digestion the body reacts by wanting to empty the contents so it is not overloaded with work.
That is how it was taught to me in A&P anyway.....it makes logical sense.