If you have a remembered username as password for a site, your web browser may autofill your username/email address into the field.
The browser is trying to be helpful, and assumes you will fill in the username and password field with your saved information.
The following should help you to stop this happening:
- remove your browser's history
- remove temporary files, and clear your cache
- restart your browser
- restart your computer
- reinstall the browser
You may find this site useful: