"This sounds like the jet effect. Are you saying the spray coming out the front of the head caused Kennedy to be thrown backwards. Just like the recoil of a rifle?"
The bullet is moving so fast, 3,300 fps, the destruction in the brain is so great that the head explodes. And if the "explosion" opening is located in the right frontal lobe, then the head will be forced away from the explosion, thus back and to the left.