The air brake system is fail safe against accidental disconnection. You can intentionally get around it if needed. Obviously they did something like that here, expecting it to be a safe setup for low speed shunting in a yard...
Most large railroad yards nowadays are "hump yards" where a locomotive pushes cars over a small hill and they roll down the though a series of switches with a computer monitoring their progress and activating rail mounted brakes that squeeze the sides of the wheels so they roll into other cars at a controlled speed to make up trains.
Smaller yards, like in this story of the runaway, are "flat switched" where a locomotive and man power manually do all this work to put a train together. Back before railroads started reducing train crews the engineer wouldn't need to get out to align switches; they had a "switchman" who would get out and align the switches as needed.
Crews also "kick" cars in flat yards which looks a bit more like humping in that brakeless cars roll down tracks on their own: https://youtu.be/zEWE3df6Q2s?t=248
[0] https://en.wikipedia.org/wiki/Lac-M%C3%A9gantic_rail_disaste...