It's the air pressure differential in the room that causes the door movement. The air pressure in the room (atmospheric pressure 14.7psi) increases or decreases once you open the first door. If the first door swings in, the pressure in the room increases due to the force you applied while opening it. If the door swings out, the pressure in the room reduces thereby creating a vacuum effect. Either way it makes the other door(s) react and it is totally normal.