Just a bug, it's happened before with other bosses in both dungeons and raids.


Usually what happens is there's a full wipe, but someone had a Raise cast on them and they stand back up instead of returning. If the timing is just right, the boss aggros them as the arena unlocks. The poor fool panics and runs to safety just as the rest of the team gets to the arena door. Boss aggro jumps from person to person as panic spreads and hilarity ensues.