WHEN MOVING VEHICLE AFTER FUELING, TRUCK WAS STOPPED, PUT IN PARK AND DRIVER OPENED THE DOOR TO EXIT THE VEHICLE. THE TRUCK UNEXPECTEDLY BEGAN MOVING BACKWARD AND THE DRIVER WAS KNOCKED TO THE GROUND BY THE DOOR. THE DOOR CAUGHT THE DOOR ON THE FUEL PUMP GUARD POST, BENDING THE DOOR BACK BEYOND 90 DEGREES AND BENDING THE HINGE, AND CAUSING IRREPARABLE DAMAGE TO THE DOOR AND FENDER, AND ALSO CAUSED DAMAGE TO A CAR FUELING AT THE PUMP WHEN IT STRUCK IT. THE DOOR DOES NOT OPEN OR CLOSE PROPERLY.