Point taken, but the four other limo passengers did turn their heads (in different directions) within half-a-second of JFK's quick turn to his left in Z-144, and Rosemary Willis did turn her head away from JFK and Jackie and towards the TSBD right before JFK's head turn, and Secret Service Agents Bennett and Hickey did start leaning over far in the follow-up car during the same time frame -- the former, having heard a firecracker-like-sound, to look around Dave Powers, and the latter, having heard a firecracker-like-sound, to look down at the pavement where the thought the firecracker had exploded.
I don't think it is at all clear when Bennett and Hickey reacted to the first shot. The picture is just too blurry to draw any conclusions. It's possible JBC's turn to the left and Jackie's turn to her right was in response to the exchange between Nellie and JFK. There could be other possible reasons for the head turns that neither of us have thought of.