I used to work with Lufthansa Cargo around the time they brought the MD11F into service. After around 2 years of operations (thereabouts) they introduced a new SOP where any void ULD (pallet or container) position in front of loaded ULDs would need to be filled with an empty ULD. This was the result of a spate of incidents where locks were discovered in the down position on arrival. The rationale was that if one of the locks failed it was not an issue with an empty ULD but the ULD will prevent the locks behind it (in front of the loaded ULDs) from unlocking. Part of that procedure was that all such incidents were to be reported and the actual lock identified.
We were told that the locks failed due to the fact that the locks get hammered during loading due to the fact that the pallets accelerate to the back during loading, this is due to the nose-up attitude of the aircraft while on ground (most of the damaged locks were at the rear). The ground staff should have controlled the speed but those lazy buggers (me included!) usually choose to help the pallet on its way! The hammering the locks received caused to them to unlock themselves.. the solution was to inspect the locks more frequently. Not sure if the design was changed later as I left around 2005.
Anilv