Sometimes PFF stats will reveal (or confirm) a clear structural weakness in a team. Last year, Cal had exactly that issue in pass protection, which was clearly reflected in weekly pass blocking metrics, whether PFF or otherwise.
But when you have one-off disasters, I think you need to either look at opponent effects or coaching/game planning/scheme issues to explain it. Consider:
Cal’s pass defense was largely fine all season*, then utterly collapsed against Oregon State. Cal’s run defense was largely fine all season long, then utterly collapsed against Utah.
*I mean, UW torched us but UW torches everybody.