There does seem to be two sources... Exotic fishkeepers who did not listen to warnings the fish would quickly outgrow even large aquariums. And chinese markets who apparently import them live. I dont think they release them, but apparently customers do.

Lest we think this is a GA thing, a few years ago a very large one was caught in a PA pond. Made the news as conventional wisdom was they would not survive that far north.

Nature is odd. Big alligator was found in a pond near Kennesaw, Ga... Far north of normal range. What they now know is they can survive, just wont breed.