Just thought I´d weigh in on this. In dogs, (inappropriate) mounting behavior is dominance behavior. I bred Norwegian elkhounds and my Alpha bitch often mounted her much larger male offspring. I promoted her top dog status (fed her first, leashed her first for walks, let her in the house, etc.) because she was by far the best trained and most well behaved in the pack.
I surmise that guinea pigs share this characteristic of pack or community status- oriented and sex-linked behavior.