Our mutt has a pseudo parental relationship with our younger cat. I’ve noticed this in a number of households that have both well-trained adult dogs (especially big ones of certain breeds) and young kittens, especially if the kitten comes into the family after the dog. I put it down to canine psychology. Dogs are very much pack animals and are at their best and most comfortable when they know their place in the pack. So as long as the humans give the dog a firm lead*, they’ll often take a nurturing role towards new arrivals. And not just kittens and pups. My first dog was very protective of me as my family had adopted him about a year before I was born.
More interesting to me is to see if the cat accept the parenting. Some will, others won’t. Cats are not pack animals at all, but since they are domesticated, some kittens seem to imprint on such a dog.
*This is something that it can be difficult to get across to some dog owners who don’t understand why they have trouble training their dog. Love your dog like your own child yes, but do not treat them the same way you would a human. Kind though your intentions may be in indulging your dog during training when it begs, you’re ultimately confusing it by sending it mixed signals, and that’s cruel.