VEHICLE WAS POINTED DOWNHILL ON A CONCRETE DRIVEWAY WITH A TRAILER ATTACHED WHICH WEIGHED APPROXIMATELY 1200 POUNDS. THE VEHICLE'S TRANSMISSION WAS IN PARK, THE PARKING BRAKE APPLIED AND THE ENGINE OFF. A WHEEL CHOCK WAS PLACED IN FRONT (DOWNHILL SIDE) OF THE DRIVERS REAR WHEEL. THE VEHICLE WAS LEFT UNDISTRIBUTED FOR APPROXIMATELY 20 TO 30 MINUTES WHEN IT BEGAN TO ROLL DOWNHILL ON ITS OWN. IT CAME TO REST IN A DITCH.