Mice are scavengers so if some babies die they will eat the bodies.
There are also other reasons as previously stated, stress or diet related, or simply a bad mom.
Since you seems to want those babies bad can you foster them?
If not leave them with her wait and see.
Make sure not to disturb her, and that she has plenty of water and quality food.
The bottom line if she cannibalized her babies without any reasons I would encourage you to feed her off.