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