BTW........

Rusty was neutered at 3 yrs old.

Ginger was spayed at 6 months old.

Mocha was netuered at 8 wks old.

Gypsy was spayed at 13 yrs old (my dad in law's dog that insisted get fixed when he moved in with us 5 yrs ago).

Mounting is often a display of aggression. We had a female (spayed) dog who mounted her stuffed animals because they were the only things she could have control over.

Yes....though I wouldn't call it aggression as much as dominance. My little fourteen pound Iggy likes to stand on my shoulders and look down on me when I am in bed. He is asserting dominance to the pack. (IN his dreams)

I agree it is an act of dominance, it is not aggressive tho it can turn aggressive. I would take your female to the vet.....could be something is wrong and she is giving off a different scent....Hope everything turns out okay. PS - it is best to stop the behavior before it turns nasty. Good Luck

Sometimes if the entire ovary wasn't removed in the spay, it can grow back v-e-r-y slowly and eventually cause her to go into heat. That may provoke Rusty into his new behavior. Another remote possibility is that Rusty was cryptorchid and the non-descended testicle wasn't removed. That can lead to a Sertoli cell tumor with increased testosterone and typical male behavior. Usually a non-descended testicle is inert, but if it became a tumor it can start to secrete testosterone. Since you didn't mention that he was cryptorchid this is probably not the case. I've seen some weird cases where the female had a UTI and it provoked male sexual behavior too. Otherwise it is probably just a dominance display as mentioned.