MY TRUCK HAS A ROTARY SWITCH USED TO SWITCH BETWEEN PARK, REVERSE, NEUTRAL AND DRIVE. MY VEHICLE WAS RUNNING AND I WAS IN DRIVE. I STOPPED AND TURNED THE SWITCH THREE PLACES TO THE LEFT. THIS PUT THE VEHICLE IN PARK. I LEFT THE ENGINE RUNNING. I EXITED THE VEHICLE AND LEFT THE DRIVER DOOR OPEN. I LEANED OVER TO LOOK AT SOMETHING AND WHEN I TURNED AROUND THE TRUCK WAS ROLLING BACKWARDS. I RAN AFTER THE TRUCK BUT COULD NOT CATCH IT AND IT CRASHED INTO ANOTHER TRUCK THAT I OWN. MY TRUCK CAME TO A STOP AND WHEN I GOT BACK INTO THE VEHICLE IT WAS IN REVERSE. THANKFULLY NO ONE WAS INJURED BUT MY TRUCK SUFFERED MODERATE DAMAGE. THIS ALL OCCURRED ON PRIVATE PROPERTY SO THERE WAS NO POLICE REPORT FILED.