From my experience with many animals, usually there is something wrong with the foal when a mother tries to kill her own baby(s) without much known cause. That, or she knew she was not ready to be a mother (aka. Motherly instinct didn't kick in as it should have). I see something being wrong with the foal being the latter from what I have read. Horses, as I have witnessed, can and will do something about another horse if it's ill.
So sorry for the loss of your little foal. :(