That's kind of sort of what I mean. For example, I've read people say that the reason his head goes quickly over to the right before the sign comes on is because he was reacting to shots.
But elsewhere, there's a more plausible reason why it happened - those group of clapping women over on the curb yelled out, "Mr. President...over here!" and that's what he did. He looked over there and then waved and then his arm kind of freezes in mid wave until the first shot.
Look through the footage of the parade before Dealey. The guy was was doing all manner of things - waving, turning, fluffing his his hair, leaning forward, talking to Connally, etc. There's even a clip where he waves and his arm stops in mid air just like it looks as he reappears from the sign.
Nothing violent starts until his hands snap up in front of face, which is where the first shot starts.