It looks like its a common problem out there. Seems to be related to the sites you are on. If a site is heavy with advertising hittin the back button is actually cycling through the sites the ads were downloaded from. You have to hit it a few times to get it to go to the actual previous page.
have't found a fix yet....but you are not alone.
Does the Internet Explorer 9 back button not work correctly for you? - Windows 7 Forums