It is a bit yucky isn't it? When I raised my last litter after weaning I fed them on a very well known and respected puppy kibble. I only bred very sporadically but it was a surprise that for the first time the puppies took to poo eating.
I took advice from a very well respected cocker breeder who advised that the puppy kibble contained so much protein that the puppies thought the pooh was just another source of food.
Could it be that your dog has come across a protein packed pooh?
My dogs don't eat pooh thankfully, but my pal's lab has a penchant for sheep's doodahs and I don't suppose we will get an answer to that.