Having read the report again, it is uncertain what the trigger event was (in a kinetic sense). It may have been the last vehicle (12 ton), or that vehicle may have moved aft (into the bulkhead) due to something forward of it shifting aft and the cascade beginning. The report was unable to conclude how it happened with certainty.
This leaves open the unknown on whether or not having this new load (three of them) to restrain ended up in a trigger event. After digesting again the extended discussion in the report on the details of loads, restraints, and this being new, the idea of cheese holes lining up leads me to guess that one of the larger loads triggered the restraint failure. They can't say with certainty, so they don't, which is as it should be.