The system is set to time out your connection after a set period of inactivity (usually 2 hours). The most common cause of logouts is an open, inactive tab that remains logged in and unused or a preview in a new window. If that's not the case, clear your local browser and DNS caches and continue using the site.
If this doesn't resolve the issue, contact Support via email or by calling 888-228-2233, ext. 307.