I've watched all the related video's on Liveleak and agree with your assessment. The bikers were being a bunch of pricks, then when they came upon the "Rage" rover, one of them had issue with the driver, and the mob rule went into effect. They surrounded the driver and began threatening him, I would have driven off too, but perhaps waited until some bikes were out of the way. It seemed the driver got freaked and punched it without regard for anyone who was in front of him.