Why do dogs hump (the air) after going potty?

Hm, this isn't a behavior I've come across! I've had humpers before, who liked humping other dogs or blankets/beds, but I've never seen it after pottying. The kicking and scratching at the ground is a way to spread their scent around the area to mark territory or communicate with other dogs, and is usually more frequent when they're pottying in an area that other animals also use. Humping in general, regardless of the target, happens for a variety of reasons. It is often related to stress or excitement, so perhaps there's something particularly interesting in your potty area, like a new dog using it or wild animals leaving their scent. Dogs are social and can learn behaviors from each other, so once one started, the others may be mirroring the behavior.