Well, Cas, I think the threads come back months prior to the original discussion because new members arrive and maybe on a slow forum day, do scour the list. I thinks it's better they do that than start new threads pertaining to the same dead subjects. Maybe this is why they scour the list also, to prevent themselves from doing that.
If an RJ member does it, maybe he had the forum tracked and thought the discussion didn't end to his liking so he post hoping it'll open back up. Again, I think it opens back up because new forum members hadn't seen it before and are curious to gain insight on what came before.