The "startle reflex",  along with some of the more primitive
reflexes--Babinski, et.al., is well documented in those with spastic
CP.  Re: athetoids and others, I'm not sure the neuromechanics would
lend itself as well.

In the spastic, it's simply our normal over-reaction to external
stimuli.  Witness, for example the extreme spasticity in affected
long-muscle groups when you place the under sudden, and unxexpected
load--the spasms are generally stronger by a manifold factor.