Sorry to hear about your loss. Was this the mom's first delivery? A friend raises goats in New Jersey. Her goats will be in an upcoming Smurf movie actually. Anyhow, the Nubian doe just delivered two beautiful babies, healthy and all seemed well. They went out to the barn over the weekend and mom was laying down, but no babies in sight. Mom got up and the babies were dead under her.
They assumed that her laying on them killed them but did have a necropsy done to be safe as they have dozens of goats. The official cause of death was suffocation.
I guess this is pretty common in new moms but none the less heartbreaking. Whether they are trying to protect them or are just not aware of where they lie, alot of it is just being a nervous new mom. My friend suspects that her doe was trying to protect them, as there had been a stray dog roaming around their area that they had run off a few times. She thinks mama was nervous and trying to keep them out of harms way.
Not ruling out the selenium deficiency but most commercial feeds contain selenium due to the fact that most farm animals aren't free range and find it naturally.