Any explainations of only the mom rats diying?

First litter was born approx 4 weeks ago, rats were big enough to be weaned then mom dies.

This last week also had 3 more moms die, 1-2 weeks after birth.

Moms are either in their own tub with babies or with one other mom with babies, dad is not in same tub.

All other rats are doing fine. Rats were "mediums" when purchased and let them grow for another 3-4 weeks before introducing them. Add another 3-4 weeks to get pregnant and theres your age. Do you think they were too young?

Sippers work, plenty of food (Mazuri 6f), they are kept in an a/c garage in freedom breeder rack.