Yes if you have done some research than you can see on every forums poeple are talking about ranking fluctations. There was one major update on 7th February, may be it is phantom update or likely they will be targetting PNB networks. Google hasn't confirmed the update yet but yes there is major ups and downs every webmasters are facing.
Because Google wants to constantly improve its algorithm. It releases updates every single day. Some might affect your site. Some might not. Sometimes, the combination of updates is what affects your site. Penguin is only one of many of these algorithm factors.