The fighter and barbarian got lucky with their rolls and took out a good chunk of healt and the bard blew up a steam engine that the monster was next to.
This is exactly why single chunky bosses drive me nuts. I once watched a rogue annihilate a major chapter boss with a single opportunity attack and some very lucky rolls.
Just an idea but couldn't the DM stretch out a fight artificially? Not the first time this happens, that could make for an awesome story. In other fights after that though, just add xx extra hp if the fight only took 1 or 2 turns.
It is a fine line but as a player I'd be fine with not steamrolling over every enemy my group encounters
So, this is a bit of a "depending on the group & situation" thing.
Fudging HP is definitely a thing DMs (myself included) do. However, with an experienced group who can get a sense for how much HP is typical for mobs at a given level, and if the amount of damage done is quite clearly far enough that he ought to be dead, it can be hard to add HP without "showing the finger on the scales" - at which point the illusion breaks and it becomes "un-fun".