Google dance is refereed to sudden changing of keyword rankings. You can rank on a keyword in top 10 results one day and another day you can go back to top 50 and sometimes even not appear in top 100 results which most SEO rank tracking tools show.

Why does Google dance happens? The correct answer is growing number of online web spam filters to fight artificial link building. Google is against excessive artificial link building and is dead against link buying. (See Google guidelines regarding link building)

So Google quality and web spam team(s) have created all these “filters” that are supposed to detect any aggressive or unnatural behaviors in link building which is the reason you may see your Google ranks changing all of the sudden. For more information on this, Coping With GDS, The Google Dance Syndrome.