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.
Meanwhile my players have been on The Galley of Death (a one shot being used as their session 1) for going on 3 sessions because they can't agree on how to escape lol
This is why I usually will add some extra HP or have some more enemies show up during a boss fight if my players are taking it down too fast. Makes the fight more challenging and fun for them so it doesn't feel like the cool boss fight was too easy.