That’s EXACTLY what happened. After Oswald fired off his two or three rounds, the Secret Service agent raised his AR-15 in response JUST AS HIS VEHICLE LURCHED FORWARD to follow the Kennedy vehicle. His trigger finger pulled back as his car leaped forward in panic, and BLAM! I believe this was a million-to-one odds accident but entirely plausible given the intensity of the last head shot.
OK, then why did the Secret Service agents in the front seat, did not report having their ear drums ruptured when that shot or shots went off???
All lead particles found inside the JFK Limo matched the box of bullets left by Oswald at the Sniper’s Perch. And the sound of the shot as you describe would have probably left the two SS agents in the front seats deaf.