it's not afraid of the leaf. the wind (combined with his snorting) has confused it into thinking it's a moving animal. to a dog, a moving animal is food. because dogs kill living creatures by crushing them in their jaws, then eat them.
yet, the leaf isn't reacting. no chase is about to happen. so, the dog is really confused that the leaf isn't expressing any kind of fear, and doesn't really know how to go about killing it.
but, idiot humans would rather anthropomorphize scooby-doo in than deal with the homicidal nature of their pets.
the dog would have reacted very similarly to a dead mouse.
