The whole incident is very confusing to watch. The BMW doesn't even seem to have a reason to slow down before the semi slams into the back of it at fairly high speed. But to make the situation even weirder, the crashes just keep happening again and again.
To give the BMW driver a little credit, he appears amazingly calm when surveying the damage afterward. But you have to wonder what this guy was thinking when trying a harebrained move like this. Check out the video to watch the carnage unfold.