If you look at the panel, Yakumo was still far from the first people lining up, so it's arguable whether the two who annoyed her were cutting in at all. All three of them at that point were only going to enter the queue, so if you run, you will obviously beat someone who doesn't. You can't really say you are already solidly in the line if there are several meters between you and the next person, unless it's the kind of line where everybody needs to be that far from each other. An ATM line isn't one of those. I never push myself between two people already in a queue, but if someone is only eyeing the line from far away, I most certainly don't feel like I'm doing anything wrong if I'm able to go in first, though of course there are always exceptions, like if it's an elder.