Final answer:
The pirates would assume that the storm caused the shipwreck.
Step-by-step explanation:
When a storm unexpectedly surfaces around the pirate ship, the pirates can assume that the storm caused the shipwreck. The storm could have caused the ship to sink or be destroyed, based on the evidence of the wreckage with the initials L.S. seen by other sailors. Therefore, the pirates would likely attribute the disappearance of the ship to the storm.