It's a dominance thing for females. She's trying to show you who's boss. If you don't want her to do that, pick her up every time she does it, look her straight in the eyes without wavering, make her stay still, and give a very firm "No". If you are consistent with this, she'll stop.
Sylphide is very dominant and has always been a humper. She never tries to hump me, though, just my husband
She acknowledges me as her alpha (although her daddy takes as much care of the dogs as I do, I do the daily walk, feeding, and grooming--so all the dogs see me as Mommy Alpha).
She has a good natured dominance game going on with her daddy. Of course, he encourages it--they play rough and tough and have a whole competition thing going on. Sylphide usually tries to hump him when they are playing and he's winning

Sylphide also tries to hump my 6'4 massive brother whenever he comes to visit!
I use a double hand clap with a no! to stop any unwanted behavior. If Sylphide starts humping (for example, trying to hump Shrek) and I'm around, I just do the clap with command and she stops.