That's because you are not checking "remember me" when you login.
Failure to check that box means your session expires after 20 minutes (or whatever the timeout on a particular forum is configured to), so when you try to post your reply after that 20 minutes, your session has expired and the forum does not consider you logged in. Since guests can't post, it makes you log in again.
TBH, we should default that "remember me" box to checked.
Imagine a pathologically idealistic, anti-religious, culturally-relativistic, iconoclastic, socially-irreverent, sociologically-progressive signature line here. Or don't.