The mob did not fire the fatal head shot. The mob did the assassination of JFK, planned, organized, executed, etc, but they did not fire the fatal round.
In the excitement, Hickey accidently squeezed off a round. I have read that he was standing up on the rear seat which would create an unstable situation. Now imagine him with the AR15 in his hand, the car accelerates or decelerates and he loses his balance. In that situation anyone would try to grab onto something. He grabbed the pistol grip thus squeezing off the fatal round.