If you are one of the S-Bahn stations where you get on the train one side, and off on the other, e.g.
Karlsplatz, then when you go to get on there will be at least one person getting off on your side at the door you have chosen. The chances of this happening are greatly increased if it is rush hour and you can see that there is an empty seat. The resulting delay is just enough to allow a sneaky German to get there first.
At the other stations, why do people crowd around the doors and then look both surprised and cross that other people want to get off first?
U-Bahn excalators are slow and often packed, and it is quicker to use the stairs if you are rushing for your connection. Adopting the same approach at the airport allows you to beat the other punters to the immigration desk.
Why are the other replies to this thread sounding like the Top Tips in Viz?