The usual reason for this are they are being timed out by the Session timeout cookie. This means that their current session is lost after 15 minutes of inactivity. Not refreshing or changing pages is considered inactivity. Composing a long post without submission, is considered inactivity.
The way to resolve this is to ensure the Remember Me? radio button is ticked. This will allow you a more lengthy session and your custom settings and saved posts will be retained for a period of up to 30 days.
One of the only caveats to the Remember Me? radio button is that (especially) if you are on a shared or public computer you must remember to log out to a stage where the Forum page confirms your cookies have been cleared.
At this stage your session, username and password information cookies will be removed from your browser. This is to ensure the person coming on after you cannot continue with your credentials.
If anyone has any queries of issues regarding this, please PM me or post below.