All while I was growing up, if a dog did the butt-scootin' boogie, it meant he/she had worms.
Guess that was an old wives' tale. Roxy had done it a couple times, but each time I checked her afterward and she just had some little pieces of dried poop in her fur. I cleaned her and she was ok. My brother's beagle does the sit-'n-spin for amusement. She sits down and her front paws go around her butt, and her butt never leaves the spot it's on. She will do this for minutes at a time. It's hilarious to watch. She is actually grinning when she does this. They always said she was one nugget short of a Happy Meal.
Maybe her bottom just plain itches.