The automatic sign-in is done at your end, by your browser, using files called "cookies" on your computer that were previously sent from here. If you do something that removes these cookies, then you must sign in manually. Many people use utilities which remove cookies from their systems in the interest of system security, but without specific configuration to the contrary, they remove the good cookies with the bad.
Alternatively, if you log in using a different computer, or even via a different browser on the same computer, it will not have access to your original cookies and you will have to log in manually.