It could be as a result of so many factors. Google could have updated their algorithm yes, but more likely in the short term your rankings on keywords will also be changing frequently depending on which websites internet users are visiting for those search terms. This could be affected by yours and your competitors content, SEO (both on page and off page), SMO etc. etc.
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.