My bitch took 4 of my puppies to a shelter and I don't get why?

I have a standard 12-year-old female that had a litter of 12 puppies 2 months ago. She has been doing well until 6 days ago she left the house with 4 of my puppies (2 males, 2 females) and didn't come back with the puppies. We found them in a shelter and the shelter workers were like "A dog just dropped them off and ran away". I just want to know why she would do something like that?