PhoenixFlyer....unsure about their reasons on NW flights. But, I have come to learn that many passengers do this on the DL 757 flights I have recently taken. Their main reason is they are seated in row 18, which are two seats together right behind the FC cabin. These seats are considered exit row seats and there is no overhead storage space available. I forget right now what is above the seats, if anything. Anyway, they get on the plane first if possible to store their luggage above rows 20. If this is not done, then they are SOL for finding space.