If too many people leave their seats and walk towards the front or rear end of the plane, it will loose balance and fall down. This is exactly what happened in Congo a couple of days ago when many passengers left their seats and ran towards the pilot’s cabin. The plane crashed. It was a silly mistake which killed 20 people. The reason for passengers panic was a crocodile, carried on board by some passenger. The creature went loose inside the plane and set panic among the passengers. Here is the animation of what happened:
Generally, aircraft use weight and balance software which tells the engine how to fly the plane to maintain balanced position in the air. The software performs a major check of weight and balance distribution before the take off. When people move during the flight, the software continuously communicates with the engine and makes minor adjustments of the position of the aircraft. That’s how planes maintain balance in the air. But there is a limit as to how many moving people the engine can handle. It was too many in the case of Congo crash.