Why does my spayed 6 yr old girl keep humping my other dog

[Dreamfrog]

Several times a day she'll mount my other dog who just lays there like nothing is happening. Any ideas why she's doing it.

No advice but all three of the female dogs I have had hump stuff.

Dpup loves to hump her favourite toy.

I've had loads of dogs who do this... boys and girls, all neutered. It can be for all sorts of reasons... boredom, over excitement, even one of my dogs would do it when he was cross with another dog.