I saw this happen in a Southern Pacific train one day on my way to work.

The crew backed up, replaced the knuckle, coupled to the fixed car, and went on to Nogales, Mexico.
My knuckle did not break; the pin that holds the knuckle closed failed. It has enough force to push it up when coupled to the train, but not enough to stay up. I can back up, couple, and be on the way to the next uncouple incident, over and over. How prototypical!
Making the video shows a second reason for the uncoupling. The big C&IM car in front of the problem car gets all shook up as it comes out of a curve with the transformer set to notch 3. I just ran the train for a few laps at notch 2. So far, uncoupling has not happened at notch 2.