Wookie does it too, but only to 'Big Dog', which is a large (bigger than her) stuffed toy dog she has had since we got her. She usually humps it after she's given it a good shaking and dragging round. The humping only lasts a few seconds. Apparently in females this can be a dominance thing. I guess she's just showing Big Dog that she's the boss!
She has never done it to any other toy, real dog or human, so I don't worry about it.