Not that uncommon an occurance. Can have several causes, a/c brakes not releasing, inexperienced tug drivers not being as smooth as they should be, fatigue building up in the pins over several months, or even the wrong pins (eg A320 rated pins in an A321 bar, but hopefully not the other way round). I've also had it happen on 737-200's, they don't have bypass pins so if there is an electrical interuption on push the steering pressurises and straightens the wheels, If the towbar is unhooked from the tug when it happens you hope your legs aren't in the way
