Ok, so why is that ? (i.e. no chains allowed)
Commercial cargo aircraft are based on airliners. Airliner floors are VERY different than military airlifter floors. The floorboards themselves are very weak. A lady in high heals can punch through them. The strength members of a commercial aircraft floor are the seat tracks. They are critical structures. In commercial cargo aircraft, cargo rails are attached to the seat tracks and all loads are distributed from the seat tracks into the underfloor structure. That is why in commercial aircraft essentially EVERYthing must be loaded on pallets. The pallets distribute the cargo loads to the edges of the pallet, and the pallet edges interface with the cargo rails. The rails provide vertical, longitudinal, and lateral restraint
of the pallet. This assumes the load is properly restrained to the pallet. If the load is not properly restrained to the pallet, the cargo rails cannot provide restaint.
Pallet locks provide longitudinal (fore/aft) restraint of the pallet in the rail. But if the size of the pallets, or the spacing of the pallets does not line up with the rail locks, then straps must be used to provide longitudinal restraint. These straps MUST be preloaded. This is usually done with a large ratchet mechanism. If there is slack in the straps and the load is allowed to shift at all, the momentum of a shifting load can easily be much greater than the static load and either the strap or the tie down will fail.
The straps must be roughly equally preloaded. If one strap is preloaded and the others are not, the preloaded strap/tiedown will take the entire load and fail. The load will then shift to the next tightest strap and overload it to failure. This continues for each strap in sequence, tightest to loosest, creating a "zipper" effect. In milliseconds all of the straps/tie downs will fail and the load will be free to move longitudinally.
I'm not saying this is what happened on this flight, but that is how the system works (and can fail) on commercial cargo aircraft. This is VERY different than military airlifters.
Hope this was helpful.