It's a bug affecting every boss fight that doesn't have the 15s area lockdown, meaning most trials, T9, T12 and T13. It happens when someone disconnects pre-wipe and the next wipe happens before the DC'd person has been booted off the server, in which case the boss keeps looping the script until the DC'd person is booted off the server, at which point the fight resets.
The instance is unable to reset the fight until all players are dead or logged out. The boss loops their script instead of respawning because bosses without area lockdown are not designed to respawn until the instance initiates it, but even if they did, you would still be stuck until the DC'd player is booted.
