The poison warfarin was once the most popular form of rat control. Eventually warfarin-resistant rat populations emerged so people stopped using warfarin for rat control. Since then, the frequency of warfarin-resistant rats has once again declined. What may be the reason for this?

Since warfarin is not used as a form of rat control, there is no selective pressure to make the warfarin-resistant allele adaptive any more. The allele likely made the rats less healthy than rats without the mutation in the enzyme that recycles vitamin K. Therefore, the allele was now a harmful mutation once resistance to warfarin was not needed.