I'm sure Mark would give a much more informed guess than I can...
It looks like it has a severe deformity, which could have been caused by poor handling. Usually, when a fish has been mistreated it would die from its injuries.
Some fish are born with these kind of deformities, so it may well be living a perfectly happy life.
My guess is; some kind of internal parasite, that has caused swelling in some of the vital organs. But that is a guess by an unqualified layman looking at a picture, not the live (or dead) sample.
I suspect that, were that the case, even an expert would need to kill the fish and do a post-mortem to find the definitive answer.
Perhaps Vagabond has some light to shed on this?