Yep - I'll be reading the linked one next too. Ta.
EDIT:From that article: "Thus if a train comes uncoupled or an air hose bursts, the brakes apply fully, automatically. The amount of braking relies on the amount the system is charged however."
So perhaps the train's brakes were not yet fully charged as the firefighters turned the engine off that the Driver had left running to charge the brake system pressure up before that pressure was reached. Or it could be that the compressor/engine was holding the pressure against a slow discharge to atmosphere.
If there are also hand-actuated mechanical brakes on the cars [which I am sure I've seen in the UK] then perhaps he made a judgement call regarding the current brakeline pressure, the time for the pressure to complete it's rise and also the number of the mechanical brakes he was going to hand-apply on the connected cars to ensure they couldn't roll on the incline.
And in the end, it could be that his estimations did not include someone turning the engine off when they did.