This can be caused when you tell your browser to “remember” your username and password for this site. If it finds a username field anywhere, it will automatically put your username in it. To solve this, you can clear your cookies/cache. Or you can disable the remember password option in your browser settings for this website.
For instructions on how to clear your cookies/cache, please visit these links:
1. How to clear your browser’s cookies – http://www.wikihow.com/Clear-Your-Browser%27s-Cookies
2. Clear cache guide – http://www.bnl.gov/itd/webapps/browsercache.asp
3. Clearing passwords saved in your browser – http://www.krbx.net/help-with-your-computer/clearing-browser-saved-passwords.html