Came across reason number 4 on Friday evening:
Particularly large wheelie cases can cause problems going through ticket barriers. Unwary travellers with these unwieldy wheelies will be unaware that the barriers are likely count them as two people, not one person with an annoying suitcase.
Alas the luggage has no ticket for itself, so the barriers come crashing in on it, jamming up that lane and forcing the unfortunates behind (e.g. me on Friday evening) to force their way into an adjacent queue.
Meanwhile, the London Underground guy, responding to the suitcase and barrier’s distress, is himself blocking up the adjacent barrier. Were this to continue, global gridlock would occur, meaning that no one anywhere could move. At all. I imagine.