Keep an eye on them when there eating… ive had more dominant/larger bass slam into/attack the stomach of others to make them spit their food so they can eat it. Had this happen specifically with 2 tems also. IMO it looks like a ruptured swim bladder.
Ive had this (ruptured swim bladder) happen once from other bass like i explained and another time during a transport netting the fish too tightly against the glass. IMO alot of bass go down during transport due to swim bladder issues from harsh netting or too shallow water in tubs.
Could have rammed a lid also… IME blunt force trauma is almost an instantaneous death tho. My cases were the same, living for a few days not being able to keep themselves up.