Don't know why dogs do that. We had a Sheltie that was a "house dog." When we lived up north we brought her with us on a visit to my parents. She followed me out into the pasture were I wanted to look over Dad's cows. First thing she did was take a roll in a fresh cow pile. I'm sure that was the first cow pile that dog had ever seen. Used some good smelling human shampoo to get her cleaned up. Luckily I got it while it was fresh so was able to get rid of the smell.