It's been many years since I've delt with loads in aircraft, but I regularly travel with heavy and potentially shifting loads with vehicles in offroad areas of the Sahara. We use the scaled-down version of the same nylon straps used for securing loads in aviation. With the constant vibration and shaking, no matter how tight one pulls the straps, within a few hours they will develop a slack due to the inherent flexibility of the material and the constant microscopic movement of the load. At some point, the slack will be big enough to permit a visibe shift of whatever is secured. These straps need constant tightening, they are checked every time the vehicle stops (naturally load shift conseqences are not as disasterous as in aviation, but in the worst case can cause a vehicle to overturn).
The accident sequence could have been initiated by load shift due to one or more loose straps, the failure of an attachment point, or a failure of a strap itself. Once the initial movement occurred, the rest was just domino effect. Hopefully a more detailed report will shed more light on these possibilities.