yeah I dont know... my other thought was he ate a bunch of pellets and maybe they swole up in his stomach??? whatever it was im adding salt to the entire tank and cutting back on the amount of food im feeding for a while.
Other than a sudden and strong bloat I can't think of anything else.
It would have to ingest a lot of dry pellets for them to expand like that inside of the fish.
Likely unrelated but my gulper catfish do this once on a while, blow up for no reasons for half a day, which I tentatively think is rinsing the stomach with water. Perhaps someone knows better.