I have heard from a previous therapist that catatonia in autism is comparable to an extreme form of "shutdown"; the brain is attempting to process so much information that it does not process much of it at all, and, thus, much of the cognitive part "shuts down." As it is the cognitive, conscious part that controls voluntary motion, to a degree, the signals to do "something" do not reach the body, so the body does "nothing."
According to this article,, medicine can also contribute to catatonia in autistic patients.
If you have the money to spare, there appears to be a 336 page book on the subject; Catatonia in Autism Spectrum Disorders..
There is another thread on WrongPlanet on the subject, if that is of any interest to you.
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I am not a textbook case of any particular disorder; I am an abstract, poetic portrayal of neurovariance with which much artistic license was taken.