If you watch it closely, he starts to lose it before he even makes contact with the rear tire of the other bike. It looked as if his front end started to go out and he was trying to catch it with his left leg and then wham. I don't know if he was braking, with the front brakes only, which caused the front end, due to being in a curve, start to slide or what. For one, they were riding way too close together. The guy directly if front of him slowed up, you can see his brake lights and that is when the other guy collided with him.
Anyhow, whatever the cause that would bite. Riding along, minding your own business and someone takes you out from behind. It almost looked like he was bowling. At first I thought he took out both bikes.