Seems obvious to me. When that car hit the side of the other car, there was no where for it to go after it slammed into that wall. This caused the car that was hit to roll over onto it's side. As it rolled onto it's side, the car that plowed into it was pushed up the side over the top of the car. (all from forward momentum) This caused it to get pinned between the car it hit and the roof. I can close my eyes and see the whole thing clearly in my head and it makes perfect sense to me.