VEHICLE WAS IN "PARK" W/ENGINE RUNNING. ENGINE ENGAGED ITSELF IN "REVERSE" ON FLAT DRIVE, WENT 122' SMASHED INTO A TREE, CONTINUED 22' REAR END OF VEHICLE LODGED ITSELF ON WOODEN DECK OF BUILDING WITH TRAIL HITCH, THEN WENT FORWARD 100' FEET KNOCKING DOWN TWO 5-6" DIAMETER TREES BEFORE COMING TO A STOP. VEHICLE HAD TO BE TAKEN OUT OF "PARK" TO REMOVE OFF OF TREES.