When you're waiting for a bus and someone else is there as well and the bus arrives, so you step to one side to allow the people on board to actually get off.
The person who's waiting there as well, usually on a mobile, seems to take this as a sign you're stepping aside to allow them on first and either gets on or steps forward to get on and only then actually looks up to see that someone else is getting off.
Happened yesterday and i actually put my arm out in front of this woman who was trying to get on and she still didn't really know what was going on.