This is one of those canine quirks that every dog owner loves. There are two different reasons for this, but in both cases this an involuntary movement that they can not control. The first reason is a reflex reaction, much like if the doctor tests your reflexes by tapping your knee: The pressure/contact on a specific area induces an involuntary reflex reaction by the same limb. The other reason is that what you are actually doing is either tickling your dog slightly or you are not hitting the right spot. The dog is involuntarily moving their limb in the direction of the "itch"/stimulation – but the feeling is not so strong to make your dog scratch it himself. I have been in the situation that this developed into the dog actually scratching the back of my hand because I wasn’t hitting the spot, but he could not prevent his leg from reacting to the itch at the same time.