we though to happen this problem, it might be the bolt do not be tightly when the worker connect the attachment and the chain plate , leading the nut be loose gradually when the chain in process of running. But you did not found that. And after the chain’s running, the nut drops suddenly, When the chain in return(position on the below location),the other side’s chain attachment (position on the above location) which the nut do not be loss from the chain plate , it will be breakage under the torsion effect .