I always think of it as a form of precession. Outside forces (in the form of opinions and predilections of the posters) acting subtly on the primary spin axis of a thread. All of those forces individually may be small but the net effect is visible
Especially in the picture threads, peeps have trouble with the fact that discussions on INGO are going to happen where they happen, there are no rails
So rails are those non-existant spoons you keep talking about?