I STOPPED ON LEVEL GROUND, PUT THE TRANSMISSION IN PARK AND STEPPED ON THE EMERGENCY BRAKE. I EXITED THE VEHICLE FOR A MINUTE TO PERFORM A MINOR TASK. I LEFT DRIVERS SIDE DOOR OPEN AND THE ENGINE RUNNING. THE END GATE WAS DOWN. I TURNED AROUND TO RENTER THE TRUCK BUT IT HAD MOVED OFF THE LEVEL GROUND AND WAS GOING DOWN THE HILL IN REVERSE. IT CONTINUED UNTIL IT HIT A PINE TREE. NO ONE WAS IN THE VEHICLE.