What you're experiencing is an Idle Timeout. Session activity is comprised of GET methods (clicks) and POST methods (Submit New Thread/ Submit Reply).
Clicking New Thread, Reply, or Quote will be a GET. However the typing of text in the text field is done locally on the browser, not the web site. The text is uploaded as a POST when you hit Submit Reply or Submit New Thread.
If the time between the GET (Reply) and the POST (Submit reply) is greater than the session idle timeout, you will timout.
If I think my reply is taking too long to draft, I'll copy it before submitting in case I timeout.