well, speed!
and btw, he doesnt need to be too fast to get into that much trouble. the contact area of the guard rail is too damn thin and the whole weight of the car pushed against it so the rail easily tore apart into the metal and other parts. if it was a wall instead, there wont be much damage to the car. i believe he was doing no more then 60-70km/h. even less when he hit the rail.