I predict: the Bump posts
In some other forums which have most recent activity threads on top, people try to keep their favorite threads active by posting meaningless "bump" posts from time to time.
This is usually done on threads where someone asks a question and is annoyed that there is no reply after a coupl of days.
Also happens when one of those "your favorite..." lists with 100 posts comes along, or when a date-oriented post like "2 months until Top Hell Iron Kitchen premieres!!!".
Moderators, I wish you the best with bump removal, hopefully if you remove them the software will reset the thread to where it should be.
I really have no problem with bumping when a person's request goes unanswered. If someone feels their post got lost in the shuffle, or they think of some details to add that might help, I think they should bump if they want. I can see how it would get annoying with those monster general threads, but if it gets bumped rather than a new thread being started I don't see the harm.