We (moderators) do have an option named "Sink". Never used it as I wasn't certain how for down it moved stuff.
Assuming that's what's going on, it seems to act like a "freeze" on a given thread's position in the index list. New posts to the thread don't act to boost it to the top of the index like ordinary threads do, and with the passage of time (and the addition of new threads and their posts), the affected thread(s) appear to sink off the first index page and gradually go lower and lower on the index listing which is continually having updates shoved in at the top. I suppose it's an effective way to gradually obscure threads that are 'undesirable' without actually closing them.
