My current hypothesis:
When someone posts to a thread with the word bump as the opening or sole word in their post, they are trying to revive interest in the thread by having it go to the list of new postings.
Or they may be wanting to attract the moderator’s attention to the thread because they think it’s bending the rules of the board too far, and someone needs to be bumped OFF the board, or the thread bumped DOWN to the Pit?
'Zat right? It’s the result of a month of puzzling.