Now, I love Ajattara and Amorphis both (my avatar has been the same since I registered on this damn site.) I have heard that Pasi got bored with Amorphis because of their mellower sound, as opposed to his other projects such as Shape of Despair, Ajattara, and To Separate the Flesh From Bones. Pasi has been known to be a shade difficult too.
He's a GREAT vocalist, though, and I hope Ajattara really takes off, they're a great band.