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.
In a cargo aircraft the palettes do lock into rails on the deck which holds them in place. However that doesn't stop the load on top from moving if not properly strapped down.