I still don't rule out the possibility that when the car took off, it jarred the cop enough that he accidentally discharged his weapon.
After seeing the video countless times on Cincinnati's channel 5, I'd bed to differ. They played the video in slow-mo up until the point of the shot so you could see the car wasn't moving, and Sam Dubose's hands were raised at the time he was shot. Only after the shot was fired did you hear tires chirp and then the car drive off. I would guess Sam DuBose had an involuntary muscle movement after being shot in the head that caused his car to flee and possibly knock the officer down.