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I tend to have one or both of my daughters with me if i'm in a queuing situation, and therefore have a backpack on (nappies, snacks, my little ponies that absolutely had to come to the post office with us). It might be that I suddenly need something from it, so take it off in an exaggerated, swinging manner that might accidentally clout someone who happens to be right up in my personal space.
It’s when you shuffle forward to get some space and they shuffle up too; right into your back pocket.