A major reason you may get logged out of the system is because the system has timed out. The will system time out and you are automatically logged out of the system after 90 minutes of inactivity.
Another reason that users can be logged off unexpectedly is by being logged in to two separate accounts in the same browser. Try restarting your browser to make sure that there are no windows running in the background. You can only be logged into the system under one account per web browser.