It actually happens often in humans as well. Often times people are pregnant with twins when they first conceive, but one ends up being non-viable and is reabsorbed. They say that 1/2 of pregnancies usually start with twins.It's a kind of defense mechanism that some animals have. When a fetus is non-viable or something goes wrong in utero the body breaks down the fetus allowing the mother to essentially reabsorb the kit. The purpose is to save the mother from birthing a carcass that could attract predators.
I saw it on a documentary, though I can't remember which one, so who knows if it's true. Just what I heard and found interesting enough to remember.