Jellyfish normally become plentiful around August, off the coasts of Korea, but now the world-wide plague of jellyfish have become more so a sign of over-fishing, pollution and global warming than of a fluke. Per the National Science Foundation, in one study notes that:

“Human-caused stresses, including global warming and overfishing, are encouraging jellyfish surpluses in many tourist destinations and productive fisheries,”

The Chosun Ilbo reports that, in August, “southern and western beaches may appear to be nearly half water and half jellyfish.” Recent jellyfish blooms have wrecked much damage on fishermen, who can not ply their trade due to these stinging menace that ruin their nets and their catch.
According to an article from 2004, in the Korea Times, filefish — a natural predator of jellyfish — have been over-fished around Korea to the point where jellyfish have flourished due to not just increasing temperatures but due to the absence of natural predators. As per the timesonline:

The problem has become so serious that fishery officials from Japan, China and South Korea are to meet this month for a “jellyfish summit” to discuss strategies for dealing with the invasion.

According to Jaunted, seventy swimmers have already been treated on the beaches of Pusan for jellyfish stings and even in a recent New York Triathlon, one Argentine athlete was killed after being stung by a lion’s mane jellyfish. (see the “jaunted” link for more articles). A New York Times article notes that jellyfish will become more plentiful:

The warmer seas and drier climate caused by global warming work to the jellyfish’s advantage, since nearly all jellyfish breed better and faster in warmer waters, according to Dr. Jennifer Purcell, a jellyfish expert at the Shannon Point Marine Center of Western Washington University.

Global warming has also reduced rainfall in temperate zones, researchers say, allowing the jellyfish to better approach the beaches. Rain runoff from land would normally slightly decrease the salinity of coastal waters, “creating a natural barrier that keeps the jellies from the coast,” Dr. Gili said.

Then there is pollution, which reduces oxygen levels and visibility in coastal waters. While other fish die in or avoid waters with low oxygen levels, many jellyfish can thrive in them. And while most fish have to see to catch their food, jellyfish, which filter food passively from the water, can dine in total darkness, according to Dr. Purcell’s research.