Thing is it was the bikers who did something in the first place, no matter how you look at it. Their mere presence on the road in such numbers, and they were sort of weaving between lanes so freely, probably freaked the driver out, as in he was probably afraid he was going to hit one of them. He may have done to antagonize the bikers, but really it's the bikers fault for being there in such numbers and not completely obeying the rules of the road.