17:33 into the video it is explained "what broke".
The door moved upwards, as is done when it is opened. It became disengaged and allowed it to be expelled from the aircraft. They have not found the four bolts that restrained it from moving and are not sure if the four bolts were installed.
Really good video explaining how the door plugs work with photos: https://www.youtube.com/watch?v=maLBGFYl9_o