I tanked HM this week and to my suprise we downed him right before we needed to move him. That has never happened before, and yes we were a bit over geared- Campaign/Hazmaat. Did they change the fight?
No, the way the fight works, he will call someone out to duel every so often. During the time before the duel but after you've been warned the duel is coming, and during the duel itself, he can be killed by being brought to 0 HP. During the time just after a duel, but before he has called someone else out for a duel, if he's brought to 10% HP or lower he will become invincible and have to be knocked off.
Easy trick to ensure you kill him without having to knock him off, bring him to 11% HP and stop DPS until he calls someone out for a duel, then burn him.