I don't know that is a great question. I know that the V also helps them rotate which bird is in front, maybe cutting the air for the rest. It would make sense if the most air resistance was at the front and all. Maybe there is the least on the longer end and the one which was just at the front goes back there when they switch out?

While we are talking about geese the plural of mongoose is spelled mongooses, I think it should be mongeeses though.