Take look at Google updates in 2013

Google Updates.

Google Page Rank on 6th Dec 2013

Penguin #5 (2.1) Update at 4th October 2013

Google Search algorithm, Hummingbird 20th Aug 2013

Penguin #4 (2.o) Update at 22th May 2013

Panda (#25) on 14th Mar 2013

                                                      Panda (#24) on 33rd Jan 2013.

Panda is Google filter update launched in Feb-2011 to end poor quality content websites having top in SERP. This can update time to time. When this happens poor quality websites loss there ranking in search result. Penguin Google launch this update in Apr-2012 to catch better websites deem to be spamming its result in search. In particular who doing spamming buy purchase links and obtaining by links exchange to increase visibility in search engine.  When this happens websites may lose their seamy back links and also lose their ranking.  G+