Both DC and Marvel appear to be headhunting him, for either Batman or Superman on one side or Wolverine with Brian Bendis on the other.
And he has to choose.
As to why he's leaving Haunt, it's to do with numbers. Not money per se, but eyeballs. Greg would like some attention for his work and he feels his art is being ignored by the comic reading masses at the moment.
DC is putting on pressure to Greg make a decision, one way or another. Expect one soon…