That'd be a hell of a hunting technique if the predators keyed into it: Driving the entire flock to crash into an object, allowing them to prey on the dead and injured.
This exact same thing happened in my front yard, except it was on a much smaller scale. The cause - falcon chasing the flock of birds into the house. It actually chased them several times until it finally forced several to hit the side. The falcon then proceeded to fly away with the dead and injured one at a time.