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.
“The egg hatched...” “...the egg hatched... and a hundred baby spiders came out...” (blade runner)
“Who are you?” “A friend. I'm here to prevent you from making a mistake.” “You have no idea what I'm doing here, friend.” “In specific terms, no, but I swore an oath to protect the world...” (continuum)
“It's a goal you won't understand until later. Your job is to make sure he doesn't achieve the goal.” (bsg)