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.