I also heard a theory (from a doctor) on TV who stated that Kennedy's muscles throughout his body were stimulated for a fraction of a second by the nerve impulses being fired when his brain was destroyed, and that accounted for his movements. I think that's bull, though.
And, if the head shot was from the back, why did his head not move at least somewhat to the front before moving back and left?
A 1988 NOVA show on PBS compared frames 313 and 314 back and forth rapidly. There is an almost imperceptible movement of Kennedy's head slightly forward (i.e., about an inch) before it snaps backward. I'm not sure where you can get a copy of the Zapruder film (maybe it can be downloaded from some site), but if you get your hands on it and are able to compare 313 and 314 back and forth quickly, you might be able to see his head moving very slightly forward -- but I admit it's quite hard to see.
Nothwithstanding the conspiracy theories that say the Zapruder film was doctored, that's one of the reasons I'm convinced the third (fatal) shot hit Kennedy from the rear.