I think you may be right. If Miller agreed to only testify about Libby, why would Rove be going back in now?
Why? To sacrifice Libby in hopes of saving his own ass
Since we're all speculating: because Miller testified that Libby told her (Miller) that he (Libby) learned of Plame's identity from Rove. (Note: pure speculation). It might also be that Fitzgerald is simply closing up "loose ends" prior to issuing a final report - not indictments.