I’ll tip the hand early and tell you the greatest likelihood is that mounting is a sexual behaviour. In fact, I think this could stand some screaming from the rooftops: mounting is sex, mounting is sex, mounting is sex! That this is not obvious to any onlooker is pretty amazing.
Let’s begin at the beginning. Fixed action patterns, or FAPs, are important behaviours that are pre-installed in animals, kind of like bundled software that comes with a computer. Fixed action patterns require no learning and are triggered by something in the environment. A classic example is a moving bit of string that triggers a six-week-old kitten to pounce. The pouncing sequence is stereotyped across all cats.
...Ethologists have coined the four big areas of endeavour under which most FAPs fall the “Four F’s”: fight, flight, feeding and reproductionEthologists have coined the four big areas of endeavour under which most FAPs fall the “Four F’s”: fight, flight, feeding and reproduction
...On to sex. At last. Reproductive behaviour is, evolutionarily speaking, the biggest and most important of the Four F’s. An animal that lacks super-duper strong courtship and reproductive FAPs doesn’t pass on its ascetic genes
...I am not sure whether the abstemious streak in North American culture whirls us, like a centrifuge, away from the S word when we see copulatory behaviours during play, or whether we’re so dominance-obsessed we co-opt nookie-nookie into some sort of power play. In any case, if you would like less of it, provide a non-violent consequence, such as a time out, whenever she does it. It could be two minutes outside the play area, or you could march her back to the car and take her straight home.