I am regularly logged out from the website and I do not know why as I do not initiate the logout. When I come back, most of my parameters are reset to default. It is annoying as almost everyday I need to re-select my general, sound, game, chat, announcement preferences. The email notifications are kept as I have chosen though.
This sounds like your browser is deleting the site data. Do you regularly delete cookies and such, or do you use any privacy plug-ins or especially privacy aware browser?