My sons dog will mount anything that's handy, other dogs, cats, chairs, people. He doesn't discriminate. I think he's just a horny old dog.
And you figured that out for yourself, before the study? ;~D
Sounds like a pit bull that jumped into our car when we were parked at an auto parts store. The dog tried to jump on my dog's back, but my dog sat on his tail with a, "Help! This guy is weird" look.