Cause and blame are not always the same thing.

Something can cause an outcome without deserving blame for it, and someone can be blameworthy for something they only partly caused. Fault is a moral judgment involving knowledge, intent, and responsibility; cause is just mechanics. Merging them makes you unfair in both directions, too harsh where there was no choice and too lenient where there clearly was.

When you are working out who is at fault, separate the two questions on purpose. What produced this, and who had the knowledge and power to do otherwise.