Chocks are indeed used in some occasions during Loco change. Here's what happened in this incident according to a senior RF. Some technical background first though.
In the air brake system on all IR rakes, there is something called a control reservoir (CR) on each coach. When the loco first attaches to the rake, it charges the Brake pipe (BP) with compressed air at a pressure of 5Kg/cm2. During this time, the control reservoir is also charged with air at the same pressure. Since the control reservoir and the brake pipe are at the same pressure, the brakes are released. The valve between the BP and the CR is a one-way valve. So even if the BP pressure...
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is reduced, the CR pressure does not change.
During the run time, whenever the LP applies the brake, the pressure in the BP is reduced. The coach has a valve that compares the pressure in the CR and the BP. If the BP pressure is less than the one in the CR, then the brakes are applied. Now, even a small difference in pressure between the 2 leads to a partial application of brakes and the train will move sluggishly depending on the difference.
Now, all locos do not charge the BP to the exact same pressure. Depending on the operating conditions, there will be small differences in the BP pressure that is created by the loco (even by the individual compressors in a loco). So, when there is a LC or an LR, if the new loco (or the compressor being used by the old loco after the reversal) has a slightly different pressure in the BP compared to the CR pressure already present in each coach, then the train will have trouble moving. To avoid this, the control reservoirs are emptied by manually operating a release valve on each coach. The incoming loco will then charge the CR and BP again to it's operating pressure.
Now in a rake that has the brakes applied, the pressure in the brake pipe is very low. If the CR is emptied at this point, the pressures in the BP and the CR become equal and the brakes are released. So normally, when the staff are releasing the CRs, the guard applies his brake, or chocks are placed on the rails, and most commonly, it is done after the loco is coupled to the rake. Apparently in this case, the staff emptied the reservoirs while waiting for the loco to attach at the other end, probably to save time, but the guard's brakes were not applied and no other preventive measures were taken. So the train started moving once, because once the CR in each coach was emptied, the brakes were released.
Note that in this case, since the CR and BP pressure are equal, and also because the BP was already emptied when the brakes were applied before bringing the train to a stop, pulling the chain was useless. Normally, pulling the chain reduces the pressure in the BP and this applies the brakes. But since the CR was emptied here, pulling the chain could not work the brakes any more.