Ah man that really sucks.
IDK, maybe it is due to the new material? I remember reading somewhere that its a completely different direction to anything they've done before, maybe it didnt sit well with him? Or the rest of the band didn't think it was sitting well with him?
Or, *drum roll* maybe he just got tired of being in a high profile band? Since he joined up (Deliverance/Damnation era), Opeth have become quite a bit more popular.
Or, maybe he just lost interest in the band. At the end of the day it doesn't matter so much, he was awesome, and I'm sure they can find an awesome replacement.
So, is he added to the new drummer poll yet?