HUNTINGTON BEACH, Calif.—Investigators searching for the cause of an oil pipeline break off the Southern California coast have pointed to the possibility that a ship anchor dragged the line across the seabed and cracked it, but two videos released so far provide only tantalizing clues about what might have happened 100 feet below the ocean surface. A Coast Guard video released Thursday appears to show a trench in the seafloor leading to a bend in the submerged line, but experts offered varied opinions of the significance of the brief, grainy shots. An earlier video revealed a 13-inch rupture in the line, but the pipe showed no evidence of damage that they said would be expected from a collision with a multi-ton anchor from cargo ships that routinely move through the area off the ports of Los Angeles and Long Beach. The slight bow in the line displayed in one video …