The SUV had forward momentum, yet it was suddenly facing sideways due to the side-back impact from the smaller vehicle. That means the tires were skidding and "

The SUV had forward momentum, yet it was suddenly facing sideways due to the side-back impact from the smaller vehicle. That means the tires were skidding and "gripping" the car at the base rather than "rolling", while the whole body wanted to go forward. Hence, it toppled and kept rolling because there was nothing to stop it.