I think it's more about that and age than anything. With the victory over Hearns, his wealth and legacy were secured. I don't think he was in it 100% after that. He was aging and probably lacked that little bit of extra hunger. It's quite common after a seminal victory. Btw (and if anyone thinks it seems like I'm looking for a excuses here, it's because I am), do you think a similar thing can be said for McCallum's first fight with Kalambay? Not that prime Kalambay wouldn't always give Mike just about all he could handle, but the fight came just after Mike's defining moment when KO'ing Curry and was the only time he was that easily handled when anywhere near his prime.
Care to elaborate on your observation? Yes, Hagler gets a lot of props on these boards but his weaknesses and faults are also discussed and on the whole I think the views held on his career are pretty balanced.