Causes
locomotive running or not, if the brake pipe pressure drops from lack of air or separating, the cars emergency air applies.
Sorta.
The brakes are applied when there is a difference in air pressure between the train line and the reservoirs. If there is a small difference, a service application is made, if its a big difference an emergency application is made. If the air leaks off "slowly" in both the reservoir and the train line, then the difference isn't enough to put the train in emergency.
Other consideration.
The danger of bottling the air on a modern cut of cars is that the air brakes may have a "quick release" feature, where when the brakes are released air from the reservoir is vented into the train line to help build the pressure in the train line quicker and release the brakes faster. The problem is that when the air is bottled, if there has been a a flow of air in the train line, either releasing or charging, and the angle cock is closed, the air, being a "fluid" can "slosh" at one end of the train line or the other creating a pulse of air pressure. If I have a 4000 ft train line with the air flowing toward the engines because the brakes have been release and I suddenly clos the angle cock the air in the train line continues to move toward the engines due to momentum. That flow raises the pressure on the end the air is flowing towards. If that pressure builds enough that the air brakes on that car senses it as a "release" of the brakes (5-10 psi) the brakes on that car may release and trigger the quick release feature causing that car to dump its air into the train line, that increase in pressure causes the next car to see an increase in train line pressure and it dumps its air into the train line and so on until all the brakes are released on the cut.
If you consider that the once the engine was shut down the train more or less became a cut with the air bottled (it was a cut of equipment with pressure in the train line, the angle cocks closed on both ends and no air supply) the inadvertent release is a possibility.
Another option.
The engines were on the downhill end. The slack is bunched towards the engines. Enough handbrakes are set to hold the train where it is. The engines are being held by the independent brakes with no handbrakes on the engines. The sole running engine catches fire and is shut down. The independent brakes bleed off and the engines roll down hill stretching the slack. The handbrakes are just enough to hold the stationary train, but the momentum of the engines is enough of a bump to overcome the brakes and start the train moving, once the train starts moving the momentum is more than the limited handbrakes can stop and the train accelerates down hill. As the cars with the handbrakes set begin to heat up, the brakes are less effective and the cars accelerate more.
And an oil car can explode if its heated or compressed enough (that's how a diesel engine works). Plus since this was frac'd oil we don't know what was mixed with it to make it flow into the tank cars, it could be the additives were volatile enough to cause the explosion and accelerated the oil fire.
Lots of different things going on. There is enough room in there for lots of human, mechanical and procedural failures, the conspiracy theories are the least likely of anything.
