I figured out why.  I have kittens and when I hold them and they get scared they lose control the same way.  So it turns out this is an innate natural reflex that developed in species as a last-ditch effort to ward away an attacker.  It is an instinctive defensive measure.  The first thing I want to do is put the messing-smelling kitten down.  It is probably the reason for the human reflex. 
Yahoo shut down the site these articles were originally on.