This sentence isn't written for elegance but for meaning. The formal cause of the accident was the mechanical separation, but that happened for a reason, either mechanical failure (which means a failure in the engineering of the aircraft, which would have to be remedied by new engineering processes) or an assembly failure (which would have to be remedied by new assembly processes). In one sentence, the author drills down to exactly what went wrong that enabled the accident to happen. Identifying that is the first step to remedying it.