One of my girls humps. It cracks me up. She wasn't spayed until she was rescued (former breeder in a mill), and we always joke (sick sense of humor) she is just doing what she had to learn to love.
While humping can be dominance related, it very often isn't. This isn't the original article I read, but here is a little more info if you are interested:
Positively Fun Dog Training: Humping: Sex, Dominance or Fun?.
