I just posted a comment on a story. After completing it and hitting "send," I wound up on the log-in page. Upon logging back in, my comment had disappeared and I had to rewrite it. Kind of a pain in the butt. Wassup with that?
It's just unfortunate that the login expired as you tried to send a comment.
Most websites have an expiry time on logins as a safety feature. It means that if you forget to logout on a public computer your login will automatically expire after a few minutes.
You can extend this expiry time to a month by clicking the 'remember me' checkbox when you first login. However eventually your login will expire again. It's necessary to keep members logging in every so often so that they don't forget their passwords. Again it's also a security feature.