After the Priests had been named, Corrupted Healing was applied on them in addition to their shield and fear resistance abilities. This meant that their healing skills caused damage and applied a damage over time. This means that if any of the Priests healed the Main Tank, the Main Tank could lose a lot of HP.
Fortunately, only one person made this mistake. After Machine Bro stopped, there weren't any more problems. The Priests were not focused on giving shields instead, and the team relied on the Druids to regenerate their HP.
During this time, the healers were under a lot of pressure because it was easy to lose players. After all, they had brought a greater number of healing Priests into this Instance Dungeon.