I find it hard to imagine there's anything at level 23 which could cause such a tantrum in terms of failing on mechanics and stuff... Sometimes tanks and healers just like to think they're better than everyone else, and if they'd not given any comment on your performance prior to that (even if it was just "stop standing in AOEs ffs") it's weird to suddenly throw a hissy fit at the end of a dungeon like that.

Maybe he didn't think you were pulling your weight against other synced-down players, maybe he'd been quietly raging for the entire dungeon over something you were doing wrong because you didn't know better, maybe he was just a dick. You meet all sorts in dungeon finder, for better or worse.