female blue tit eating chicks in nest?

In the nesting box with a camera I have avidly watched Mummy blue tit make the nest , lay 6 eggs one by one ,all hatched successfully. Mummy blue tit [Daddy somewhere on the fringes] have fed these fledglings for the last 4 days. Yesterday only 3 chicks were alive and I have just witnessed Mummy BT pecking away at dead chicks.It looks like she is eating them then she drags them out of the nesting box.

Now she has none and I am very sad. Does anybody have any idea what might have caused the chicks to die and, is she eating them?

Note: Mummy BT does not seem as traumatised as I am she is nestled down in her nest presumably about to start over......