I would think stress, even if there weren't any overt signs until it was too late. Stress is a really big deal and plenty of fish are hooked, look great and then promptly die when returned to the water. The air bladder suggestion is also valid, if he was down deep when you caught him he basically would've gotten fishy bends.
(I guess I gotta be that guy now) Another thing to consider before trying this again is that a 75g tank is far too small for a LMB. I don't know if you've got something larger set aside or not as these are the only posts I've seen from you but you should be looking at 3-400g minimum for an adult LMB.