I do not think that it was any action taken by you brought back into ranking. For past few months i have been experiencing same ups and downs in google ranking. My site disappears all of sudden for few days and i can see clear re-indexing by google and then it comes back to business again. Sometimes it took even more than 20 days to regain the ranking.
As far as I can notice, it is happening with the dynamic pages or the pages that are user generated. Google keeps re-indexing dynamic pages/ user generated pages and results after a certain interval.