Have you had players being automatically checked out even if you, nor them, have performed this action? This can sometimes happen if the player uses an e-mail provider with a very strict spam policy. Some spam preventive software will "click" every link in an e-mail to determine if there are viruses. The last link on our "Game day" reminder e-mails is an "OUT" button, which would cause the software to automatically check the player out.
Unfortunately, the only solution at this time is to ask the player to use a different e-mail provider, such as Gmail.