If the sidebar is floated to the right and is shorter than the main area the footer then is required to jump up into that available space, like below:

To fix it, you can 

#footer {
  Clear: both;
}

or add an empty div before the footer block with the style set to:

<div style="clear:both;">

or, even better and more clever using a CSS pseudo selector (:after) to clear the floats. It’s extremely helpful in situations where making changes to the stylesheet is the only way to fix the problem.

.floatblock:after {
  content: ".";
  visibility: hidden;
  display: block;
  clear: both;
}

/via CSS-Tricks/