Normally load shifts happen on takeoff where you have a high nose up body angle and acceleration at the same time.
I assume the load is secured from moving in multiple directions, depending on the items in the load. I could see the case where a strap holding the load from moving forward was improperly attached, or perhaps failed in transit.
That being said, I did hear one statement that the load contained no large items like vehicles being transported. Most of the items were probably in containers, or on palettes. I would assume those lock into rails in the aircraft body, although how those locks work I have no idea; they might also be subject to directional forces.