From Cecil Baird:

All kinds of possible causes including overfeeding with too much feed in it's gut causing pressure and obstruction of the pneumatic duct and air bladder where excess air is released. Not all fish have this duct, but trout are one the few that do. Although his water is now 57 F. he may have been feeding in much colder water where digestion is much slower?

Also an overinflation of the swim bladder can be caused by an infection of the swim bladder caused by a virus or bacteria. And another possibility are gill parasites. If gill parasites are prolific enough to reduce oxygen intake, this can also open up the fish to an infection.

Sometimes over inflation of the swim bladder is associated with poor water quality, which opens up the fish to subsequent stress and infection. No idea how big the body of water is but, maybe too many trout for the body of water, and too much feed for that body of water?

Would be interesting to see what the ammonia levels are for that body of water. (He could probably pick up an API color test used for aquariums somewhere.) This early in the season the nitrification cycle can't always catch up with heavy feeding in a small body of water due to the earlier colder water temps.

Or it could be simple as a physical anomaly with that particular trout, and he will never see it with the other trout. I.e. neoplasm, which is a benign or cancerous growth messing with the swim bladder or pneumatic duct between the stomach and air bladder.

Hope his helps. Without a close examination by DVM specializing in fish it could be a lot of things. I'm just throwing out possible causes. If I was a betting man I would guess this or very few trout will have this problem

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