What happens is, another customer comes along, sees three cashiers and one queue (in the middle) and decides to join the "empty queue" in front of the cashier at either end.
Of course they know there's only one queue and that they are queue jumping, but they calculate that they can pretend they didn't know.
I assume there's a cultural factor here. I'm in the UK and have seen this many times in one store nearby, but I imagine that in some parts of the world you would be beaten or shot for it. I have never seen anybody complain about it, they just temporarily disperse into three queues (which is awkward because there is not really enough room.)