Basically, when a topic breaks a forum rule, or is pointless, or is a duplicate thread, a forum moderator will come along and “Lock” it. In other words, no one can post in them, and they quickly drop to the older pages.
Moderators lock threads if they find them to be inappropriate, Obscene, If it contains Inappropriate Content Racial Abuse, Swearing ETC, If it is a duplicate thread, If it has no point or is useless and So on they will be locked.