Jo we too have had a couple who eventually just seemed to go off the deep end despite the same uniform care and conditions afforded the rest of the rats - and all the other mommies are almost congenial. The last couple times it happened seemed to mirror your experience - a partition would have to be used just to be able to reach in the cage. In both instances after noticing she was destoying her litter we broke down and redistributed them to other moms who had babies of the same size. The seem to be rally great about being universal moms - regarless of genetic parentage.

I am not sure what was at the root - in both instances (coincidence I am sure but they were both albinos) they became food. I have seen the mothers who did not exhibit pregnancy rop litters while still int he cage with a male - and know the males attempts to mate can set the mothers into a defensive frenzy - but in these couple of cases where they are in their own cage with their litter and nothing else I am not sure what to make of it.