How does passenger unloading order prevent plane tipping?
Apparently, there have been a number of incidents with 'tail tipping' in case of Boeing 737-900s and 900ERs. In these situations, the plane 'tipping' on its tail can be prevented by two methods:


Unloading the passengers starting from rear to front.
Unloading the (baggage from) aft cargo hold before passengers were allowed off the aircraft. 


There is also some indication that the 737-900s are loaded in the front rows (1-17) first followed by aft rows, while the unloading takes place in the opposite way.