This is not usual. Sometimes they will die if the mother gave birth to soon after her last litter. Then she would not have enough milk for the new ones and they would die if not nurished soon after birth. It is good to give the doe at least 2 months before breeding again. If this was her first time, she may just not have taken proper care of them(being young herself)Also if she was disturbed by being moved just before birthing she may have felt it was not a safe place for them and killed them. I had one doe that never would care for her babies altho. she was cared for properly.