What knocked him out of his seat?
I would suggest that might have been the cause of it all. And that by the time that had occurred, it was likely that it had also knocked the car completely out of control, that it would have been unlikely that the car could have been brought back under control in time to have not hit the people waiting for the bus.
BTW- I always wear seat belts. I learned that early in life while flying aircraft. This reasoning does apply to aircraft in a way that it doesn't apply to cars. It's a matter of timing.
Turbulence can throw a pilot out of his seat, when if he wasn't thrown from his seat there would still have been time to recover, and bring that aircraft home.