I think this thing takes place in automatic cars but it is very rare. What I think happened is the driver left his car engine running and the automatic gear slipped from parking to reverse. The car then moved. It is lucky the car hit fences and stopped otherwise the longer distance it would have traveled, the more speed it would have gained. However, with such incidents having taken place in the past, car manufacturers have put in a locking mechanism in all automatic cars. Possibly, the locking system failed and with the hand brakes not having been pulled, the car just drove down the road all by itself.