Board Thread:Episode Discussions/@comment-36996724-20180923165524/@comment-1458758-20180923175348

Well the trap was done basically so his shark could escape with the intel, from what we see, soon after, they're out of the water, but the sharks are still flopping around, so I imagine the technique was designed to trap everyone nearby when the scroll was opened, but obviously wasn't intended to trap so many people, so it probably ran out of strength,releasing the water prisons, but the summoning still remained.