Ever had to swap a healer in the middle of a raid? Maybe he requested it. Maybe he's experiencing connection-related problems. The most common reason I can think of is because he's not able to handle his position well at all. I'm also guessing you tried to switch healer assignments in case he kept striking out. Clearly that didn't work, because you're in the process of bringing in a new player to replace him.
When doing this, you need to talk to the player twice: once to signal his stepping out, and then once again after the raid for a diagnostic on what he messed up on.
Filed under: Raid Rx (Raid Healing)