Wow, that thing you found on Yahoo is really good info. I'm glad you posted it!
Yeah, in my experience the dog scoots across the rug when its butt is itchy. That's one part it can't reach with its paws or mouth. Ya do what ya gotta do, right?

It could be for any number of reasons. As already suggested, the dog could have worms. You said "puppy", right? All puppies have worms. It's just one of those gross but unavoidable things. They usually need to be periodically de-wormed until they're grown up. So if it's a puppy, that's the most likely thing. Some worms, like pinworms, can even be missed in a fecal exam and still be causing the itchiness. Luckily they're pretty harmless except for that one obvious symptom. Fleas or allergies could have something to do with it too. If the puppy's little butt cheeks are full of bug bites or the skin is irritated, they'll be scratching like crazy.The anal sacs are a possibility, but if it's a young dog those parts probably aren't even developed yet.
Take a fecal sample to the vet and see what's up. I'll bet it's worms. Worm medicine isn't typically expensive, and it's good to find out because some types of worms do more damage than others.