It's not just bikes. I had to walk for two weeks recently, and found the same pattern of "let's catch this guy off guard and make him jump or something," presumably as a way of feeling powerful in a car, with the speed to get away before someone grabs you and wrings your neck, plus the armor of the steel body and the fact that your victim doesn't know where you live and therefore can't come and tell your mother and get your driving privileges revoked.