Intact female dogs may hump other dogs or whatever else is available when they're in heat, but it is not unheard of for spayed dogs to engage in the activity. Your spayed pet may have learned while she was intact that humping feels good, and does so as a masturbatory behavior. This isn't the only reason, though -- in fact, her humping may have nothing to do with sex at all.
Reasons for Humping
Some dogs hump people, furniture and toys when they become particularly excited, lacking a sexual urge, because they don't know what to do with their sudden burst of energy. Similarly, your dog may hump things as a compulsive response to stress. She could even hump other animals as a form of play, or a way of asserting dominance over them. Humping may, however, be her way of coping with a medical problem, like a urinary tract infection or an allergic reaction around her genitals, so consult with a vet if you notice this behavior.