Even though a wheel or axle may seem to be completely rigid, there is an awful lot of torsional energy stored in one that is braked to a stop -- especially if the final braking was severe. My guess is that this energy was held in by the final braking, and was only released when pressure in the hydraulic system bled away after the engines were shut down. It probably took a couple of minutes before enough force to overcome the static friction of the tires was accumulated, but after it did, the roll began.