After a certain amount of time the system will log someone out for inactivity if they don't do something. It's probably reaching that time while you read the thread. Clicking the stay logged in button is an option, or if you only access the internet at public sites, refreshing every so often, or copying a reply before trying to post it to make sure it's not lost to a log out is an option. If you are logged out, you could just paste the reply after logging back in.