It just happens Jay. With any animal that gives birth, stillborns can just happen. One baby or 20 baby creatures not all are always meant to make it. Some never start breathing either due to a problem with their health or the mother rodent not stimulating them or getting them loose of their birth sac/cord quick enough. Sometimes they aren't strong and the mother animal just knows that and doesn't bother with them at all (letting nature take it's course type of thing). Sometimes it's an inexperienced first time mother whose just overwhelmed in the birthing process and doesn't attend them all quick enough or lays on some while birthing/nursing others. Basically it just sometimes happens.

You may see the mother's kick these dead pinks out of the nest or even eat them (recycling them basically as gross as that sounds for us humans, it makes sense for the rodents). I never worry about stillborns unless it's a high percentage of the litter or happens a lot with a certain female. I don't get many of them but I suspect there are some I just never knew where birthed (the recycling thing).