If he slammed into the back wall, it would be very difficult for a van to accelerate quick enough to cause that alone. Now if he slammed into the front wall of the back area, then possible a massive braking could have caused it.
According to the other passenger, the ride was not rough or erratic. I think the guy was trying to injure himself and seriously over played his hand.
If I were making police policy, I think I might change the patrols so that only black officers patrolled and made arrests in black neighborhoods. It would be hard for anyone to make racism an issue with that policy in place.