All first time comments by a reader automatically are moderated. Once a reader’s comment has been approved their future comments should not require approval for posting. This is a protection against spammers.
For some reason, sometimes WordPress boots an approved reader’s comment into moderation even though it shouldn’t. I don’t have any control over this. It just happens. So, if it ever happens to you and you wonder why all of a sudden you’re being moderated…please be patient because it’s not me….it’s WordPress. 🙂