The drop in Clinton's approval is a significant dip from her ratings while she was Secretary of State. The numbers are surprising as Clinton is widely considered to be the Democratic favorite for the 2016 presidential election. Since she left the State Department she has been working on her book and been the subject of endless speculation about 2016.
She has not officially announced her bid for the presidential seat but potential GOP candidates including Ted Cruz and Rand Paul are well on their way to the campaign trail. On the Democratic side, Vice President Joe Biden has also indicated that he could possibly start a campaign.
Clinton's approval may see a boost, though. Clinton recently announced the title of her memoir and has confirmed she is "thinking about" a 2016 presidential run. The dip in approval is likely due to the fact that she does not currently hold office.