Today it happend few times. we were fighting Lord Mardrom in arena. Someone died and respw 2 continued fighting. Then Mardrom suddenly reseted without any reason. No one from fighting persons died. I saw bosses reset only when everyones dead and barrier is off then so we can enter arena again.
This time when Mardrom reseted without reason barrier went off also so those who died could enter. I just stood in the arena and was like huh? very unpleasant when he was half life.