Searchenginegenie.com

Google Penguin has on going changes in its algorithm and filter those websites that are involved in link schemes. The fluctuations in ranking positions is the effect and also the revert back that you said getting back in their old position. Expect many changes in the future because Google Penguin is updating from time to time.

What happened is both penguin got updated and search engine optimizers began to take penguin into account and so filtered and removed worst of their links, as this is the only working penguin recovery method