A major reason a user may get logged out of a site is because the system has timed out. After 90 minutes of inactivity, the system will time out and you are automatically logged out.
Another reason that users are logged out unexpectedly is due to 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.
Admins that are managing SLM and GLM sites will also experience an automatic log out when they switch between the two systems while using the same browser.