I think some mothers just dont make good mothers at all. But since we know she raised other litters prior to this, it makes me think she was stressed for whatever reason despite all the great things you do to minimize stress or... there may very well be something wrong with the babies that you cannot detect with yur eyes... rats will kill thier young if they "feel" they are week and unable to survive... but I have wondered what it is that makes them decide to do it cause I have had babies that outwardly appeared fine and I finished raising and they did fine... guess we will never really know. But I think you did the right thing by getting rid of her for the reason you have, just replace her with another breeder female.