I would suppose a 10 to 20 pound reduction of train-line air would be typical for stopping a 72-car, 10,000 ton consist on 1.2% grade. After stopping the train and tying it down, the locomotives' independent brakes would have been left applied to ensure the train remained stationary while the sole operator/engineer was outside of the lead unit applying handbrakes on trailing units and cars. After performing that work, the O/E would necessarily have released train-line air brakes during a push-pull test while monitoring the consist for no movement. During a push-pull test, it's likely the locomotives' independent brakes remained applied. Indications are the train-line air brakes were not set up afterwards. Of course, the locomotive event recorders will reveal what actually occurred as the O/E tied down the train.
The key point to observe here is locomotive independent brakes are a "straight air" system that loses effectiveness as main reservoir air pressure declines. The loss of locomotive independent air brakes resulting from main reservoir pressure bleed off must be a significant factor leading to this runaway incident in Lac-Megantic. After fire department emergency responders in Nantes had performed shutdown of the lead unit's engine and its air compressor, the eventual loss of main reservoir air throughout the five unit locomotive consist caused 30 axles of operative air brakes to dwindle down to 5 axles with handbrakes (a sixth, or less, of the braking force holding the locomotives).
Reportedly 5 handbrakes had been applied to cars in the consist. Add 'em all up; 30 locomotive axles, plus 10 or so car axles, totals 40 or so braked axles preventing the train from moving. Lose the independent air brakes on the locos, and this total declines to about 15 axles. That's over half of brake forces lost over an hour or two of time that air bled out of the main reservoir and MU distribution system.
So, this doesn't appear to me as the work of eco-terrorists, pipeline sympathizers, saboteurs or vandals, nor inadvertent operation of locomotive air brakes by fire department responders - just bleed off of the locomotive independent brake system that was primarily holding the oil train from rolling off of that hill above Lac-Megantic.