Another common reason dogs hump the air is to assert dominance. Commonly dogs hump the air above other dogs to show the alpha. This is also a behavior often seen when strangers enter a home.
There are quite a few medical conditions that can lead your dog to hump the air. Dogs that perform this behavior may be experiencing pain in their genital area.
Another prevalent reason for dogs to hump the air is displacement behavior. Displacement behavior starts with an intense feeling of some kind that then manifests as an unrelated behavior.