And the bullet exiting the front caused those behind JFK to be showered with his brains?
Yes, the motorcade was moving forward, and the people immediately behind Kennedy thus drove through the blood cloud that was still in the air for a few seconds.
You'd get the same effect if the motorcade were standing still and a strong breeze were blowing from "front to back" -- the air would carry the blood spray back onto the people behind Kennedy.
And the spray in frame 313 *very* clearly shows the blood and brain matter spewing forward from JFK, as a result of a shot from the *rear*, no matter how much conspiracy nuts may try to obscure that by hand-waving about specks of blood on a motorcycle officer immediately behind JFK, or his head/body movement subsequent to the shot, etc.