The mother cat keeps carrying the kittens away, probably because she feels the surrounding environment is not safe and wants to take the kittens away to a safe place.
It's also possible that the mother cat is grooming her kittens, which is a natural instinct for mother cats. They use their saliva to clean their kittens and keep them clean. In addition, to train their kittens' survival skills, mother cats will carry them in their mouths to guide them in learning how to climb, jump, and other skills.
