Brandon Green comments

On most trains, doors are controlled by the conductor or driver and there are some buttons that open and close the doors. However, the doors can also be locked/unlocked so that the buttons right next to the doors themselves can operate the doors as well. On some trains, however, the doors can be locked/unlocked individually at the door, and this decouples it from the rest of the system, so when someone closes all doors, that door will remain open. I'm not sure specifically how these doors operate, but likely it was stuck open until staff could go to the door and lock it.

Many trains are also incapable of being driven if any door is detected open, however as the door was manually unlocked, this may have disabled the safety check for that specific door. It is disappointing that this happened, but I am glad there were no injuries and the safety can continue to be improved in the future. It is already impressive that these trains have started and stopped millions of times, yet it took until now to have a problem.