There could have been any of the reasons Skipper put up there, as well as something wrong with that pup, as you said.

It's impossible to say from here. How many other pups and how is she being toward them?

Is the mama wormed and healthy? If she is skinny, weak, or undernourished (including being full of worms) she may eat them to nourish herself.

Post a photo of the mama, her pups, and your setup ... and also tell us what you're feeding her ... and what you're worming her with ... otherwise we're playing guessing games with no real information to go on.

Jack