DURING A MINOR INCIDENT WITH LOW IMPACT AND AT A LOW SPEED, BOTH FRONT SEAT SEATBELT PRETNESIONERS FIRED OFF, LOCKING BOTH BELTS COMPLETELY AND FILLING THE TRUCK WITH SMOKE. NO ONE WAS IN THE PASSENGER SEAT WHEN THAT ONE ENGAGED. I WAS BACKING UP IN A SHOPPING CENTER PARKING LOT SLOWLY AND DID NOT SEE A CONCRETE SLAB BEHIND ME DUE TO THE ANGLE.