Something doesn’t seem right with this explanation. In most air brake systems if there is no air pressure the brakes are ON or applied. It’s the air pressure from the compressor that releases the brakes. If the engine is off the air compressor isn’t generating air pressure which would mean the brakes are locked ON. In big trucks and even in my motor coach if the engine isn’t running to produce air pressure to release the brakes it won’t move.
Though not proven at this point, there are background rumours that this incident may have been deliberate eco-sabotage by the anti-oil kooks.
Several groups in the US have already engaged in actions they use to stop the movement of North Dakota oil, and have not been shy about it - they call it “Fearless Summer”. An extremist group called 350 Maine set up a blockade 2 weeks ago - Maine is next door to Quebec.
Stay tuned.