Hillary's numbers falling

Are Hillary's numbers falling because she is not as likeable as a candidate, or because people disagree with her ideas, or because people have a sense that she has made mistakes?    

 http://www.washingtonpost.com/politics/democrats-finally-get-a-contest-as-hillary-clinton-draws-challengers/2015/06/03/95fefaea-0a12-11e5-95fd-d580f1c5d44e_story.html