Alternative narrative: Customer demand shifted to cars with higher mileage. Auto makers stopped resisting higher CAFE standards because the minor increment was meaningless. See also: The ongoing debate over minimum wage laws and unemployment.
Inferring causation from correlation is not "pretty simple", but a lot of poor decisions are made under that assumption.
I can tell you for a fact that at least on large diesel truck forums when gas/diesel hit $4/gallon the number of used trucks being posted for sale was staggering. The number of people who couldn't afford to fill their trucks up and still make a payment was almost comical.
Inferring causation from correlation is not "pretty simple", but a lot of poor decisions are made under that assumption.