Seeing as you've just said it yourself, that you can't see what's going on, and the rest of us are assuming that the situation must have felt serious enough to warrant his actions, then I'm really not sure why you can't see everyone else's point of view.
And once again, having been on the receiving end of biker rage on more than one occasion, I'm going to continue to assume that most bikers are indeed idiots, as it's safer to assume that IMO. There is NEVER any need to form a group large enough to take over a three lane highway for hundreds of yards, I'd feel intimidated even if they had done nothing wrong.