Parent finch (mum) attacking baby finches?

i have 3 baby finches which are about 2 weeks old, the mum has started pecking at them, it looks like she is pecking out the down on the babies, she has slowed down in feeding them, just wondering why she is doing this? there is also blood on one of the babies, she is pecking at them so much that one of the babies fell out of the nest today, please help?