Diamond does it to Ebony a lot. (Diamond has some ACD in her) Diamond is lucky Ebony will put up with it. She knows better than to try it with Jada. She only does it when her and Ebony are playing. I dont get it myself because Diamond is on the bottom of the pack here. I tell Diamond to quit and she stops.

*edit to add*

I went to google and found this article. It might help you. There were a lot of different articles on the subject.

http://www.petplace.com/dogs/humping/page1.aspx