I am sorry for your loss. It's never easy when we lose one of any age.
It is possible that the baby had a problem (cardiac/circulatory as Pam suggested (?)) that the mother was aware of and that is why she rejected him. They seem to know. If he had a heart problem, it may have prevented him from growing normally.
From what I've seen mothers will ostracize or refuse to care for a baby that is sick, injured or "not right". Their instinct is to care for the ones with the best chance of survival. They will allow the baby to move away and die, so it is not a "danger" to the rest of the litter. (In the wild, the smell of a dead kit can give away the location of a nest to predators... or bring in flies or bacteria that can infect the other kits.)
Hugs to your other babies...