I don't know "how" it happens but I have an understanding as to "why"-- in a wild dog pack generally the alpha bitch is the only one to be mated and whelp pups, but she became alpha for a reason-- she is also generally a terrific hunter. The pack needs her hunting skills to thrive, so other females in the pack will lactate so that they can raise the pups while she is out working. Learned this on that In The Womb: Animals National Geographic special from sometime this past year (I bought the video b/c it has 4-D images of puppies in utero). So not too crazy that some, but not all, domesticated dogs do this as well.
I currently have a litter of 6 papillon puppies who are 7 weeks old tomorrow. Weaning to kibble is going well but they still nurse a few times a day. Their mother, the best mom ever LOL, has been regurgitating about .3 of a cup of food about twice a day after she nurses them, this has happened in the last week only. No vomiting at any other time. My theory is that she is unconsciously working to wean the puppies. Isn't mother nature fascinating? Of course I pick it up (usually before the puppies mob it) and try to explain to her that I really don't need the help as their kibble eating is going just fine
