Mass die-offs of single species is an interesting and not well-understood issue. It's kind of fun to speculate what might have caused these jellies to die off en masse. A virus? A severe lack of oxygen in a "dead zone"? Perhaps a high concentration of near-surface jellies encountered an ocean-going ship and its massive prop killed millions of them, the way the grain ship props kill herring by the thousands at the port of Tacoma, or the way the ferry boat terminals near Seattle are fabulous locations to mooch herring baits in the wake of the ferry. Perhaps seisimic activitiy? On the west coast back in 2009 I think it was, we had a HUGE die-off of giant squid, which coincided with a pair off fairly significant deep-water offshore earthquakes, and giant squid washed up from mid-California all the way north to almost Canada. Perhaps military submarine propulsion killed these little buggars off? Perhaps an offshore lighting storm electrocuted a bunch of them as they floated near-surface?