Keep in mind the simple fact that she has ethics might be a valid reason for it to be AF withdrawing. As we saw with Brad Conway, when backed into an ethical corner typically a lawyers only option is to withdraw. if she is seeing behavior that runs counter to her ethics she can't simply babysit it, or even report it.
I still lean towards the departing party being CM, simply because he seems to be winding down his participation, beyond the bomb throwing and JB defending. if this is the case it may mean that JB will be in for a world of hurting. I somehow get the impression that while she is nice, classy and subdued, Ms Finnel would not be playing by JB's rules when she is the required DP attorney. At the very least she would probably be having some private meetings with KC, and hopefully have the long needed "come to jesus" conversation about her actual chances at trial.
Of course one has to wonder if maybe that conversation has already been had, and this departure may be a forerunner to some sort of plea deal that would obviate the need for that particular lawyer?