I just lost about an hour and a half's worth of work when I was told that I needed to log on after attempting to submit a long post. So I logged in again, and all my work had vanished. Would I have even been able to post in the forum at all without logging in first?

Hopefully I'll know better next time, but what's this about? Is it a security measure, or just a bug?

Unfortunately I don't know what happened. It is probable your session became invalid, which in effect logged you out. This can happen due to many things, including a significant change in IP address, or your browser cookies being invalidated.