Relating to the question of why the plug opened on this particular flight an not any of the earlier ones:
As I understand it, while the springs provide an assist to lifting the door upward, they aren't sufficient to do it on their own. The weight of the door/plug is near, but still greater than, the force of the springs. The door needs someone to manually push it upwards to open. If this is true (and I stand eager to be corrected) then what kept the plug in place all this time is simply gravity. What caused the plug to finally move upwards may have just been a peculiarly angled jostle of the plane while airborne, imparting enough force in the right direction to move it past the stop fittings, and with enough pressure difference to then force it out.