2013 Nissan Armada POWER TRAIN,ELECTRICAL SYSTEM

TWICE, WHILE I THOUGHT THE VEHICLE WAS IN PARK, IT BEGAN TO ROLL. THE FIRST TIME I CAUGHT IT RIGHT AWAY AND IGRNORED IT. THE SECOND TIME WAS TUESDAY MARCH 11, 2014. I ARRIVED AND WORK AND PARKED. IT WAS DARK. PUT THE TRUCK IN PARK, TURNED OFF THE ENGINE AND LIGHTS; WHEN I OPENED THE DOOR, I COULD HEAR THE WIND RUSHING BY (I WAS PARKED IN A PARKING LOT (VERY SLIGHT DECLINE.) ANYWAY, GOT BACK IN, SLAMMED ON THE BRAKES, BUT STILL MANAGED TO HIT THE BUSHES (SMALL TREES) IN FRONT OF ME. WHY AM I ABLE TO SHUT OF THE CAR IF IT'S NOT IN PARK LIKE IT SAYS? *TR