On several occasions while in reverse the car braked hard and violently for no reason. This caused a sudden jolt to me and the passengers endangering our safety. Also, this condition can cause an accident if Im trying to back up to avoid a collision and the emergency braking system ds not allow me to do such. The vehicle has not been inspected by anyone else and no warning lamps messages or other symptoms of the problem appeared prior to the incidents. This condition first occurred with about 300 miles on the vehicle.