The automatic logout logic that has been recently added is that you get logged out after a certain amount of inactivity. This is described at . If you are inactive for say, 30 minutes, you get logged out automatically.
However if you are an admin user (like I normally am), it is not uncommon you have /cms tab open and /cms/console. Now, if in 1 of the two tabs you do not have any activity for 30 minutes, your http session gets invalidated causing a logout...Obviously, this is a bug: It should be about user activity in one of the tabs.