Not clear from the -typically lousy writing! - what happened. Obviously, the coupler broke completely, or lost its “locking pin” and opened up to release the cars behind - but the did the hoses remain hooked up and that was what was pulling the cars behind as the train slowed?
The following car(s) would not have automatically tripped on their brakes unless the air hoses themselves were broken. So what did the engineer do? How did he know the coupling opened up - a phone call from behind in the back cars?
My hypothesis #1: Something on the track engineer saw and started to slow but still hit. Hit caused bumping and jarring that broke hitch, momentum allowed back car to stay close enough after hitch broke and engine continued breaking to allow air hoses to stay connected.
My hypothesis #2: Engineer was sufficiently alert that at the first sign/sound/jarring of a problem he immediately started breaking and allowed back car to stay sufficiently close.
I don't know but you'd think there's some sort of electronic sensors connected front to back to alert the engineer. Of course if the air hoses stayed in tact maybe the electrical connections stayed in tact as well.