This is commonly due to a Background Process that runs after a predetermined period of time and is specified by the operation, this could range from 10 minutes to 24 hours or longer.
When the process runs it looks at the users that are signed in and performs a check to see when they were last active on the system. If this is longer than the specified time, then the process will close the users session and they will be required to log in again to continue using SnapFulfil.
If you look at the below screens, you can see on the left that the session was started at 08:30 on the 19th and then terminated at 03:39. This coincides with the process running on the right. 
The reason for this is to clear inactive users from the system and stop them from locking up licenses.