CAR WAS SITTING IN PARK AND STATIONARY WITH ENGINE ON IN OUR HOME FLAT DRIVEWAY AND DRIVER GOT OUT OF CAR TO RETURN TO HOUSE WHILE CAR WAS IN PARK. THE DRIVER WAS GONE FOR 20 SECONDS AND AS DRIVER WAS RETURNING TO CAR THE CAR STARTED ROLLING BACKWARDS IN DRIVEWAY FOR 150 FEET UNTIL IT HIT TREE AND STOPPED. WHEN DRIVER RETURNED TO DRIVER SEAT THE GEAR SHIFT WAS IN REVERSE.