In B+W Ash's Scraggy gets easily overexcited and dragon dance is described as a 'vigorous' dance and this would fit with Scraggy's character. It would make sense for Scrafty to learn it since it is Scraggy's evolution.
Dragon type moves are thought of as raw magic and Scrafty can learn other magical moves such as rain dance, sunny day, zen headbutt, focus blast, Dragon pulse and dark pulse so it would make sense for it to learn Dragon dance.
Lastly, Scrafty is partially based on some sort of lizard or snake; it sheds its skin. Snakes and lizards are commonly associated with dragons.