My guess (complete amateur speculation) is that an insane number of people got hooked on opiates due to over prescribed pain meds. The medical industry then stopped doing that so the conveyer belt of new addicts has been turned off. I think the drop in deaths is a trailing indicator for the lower levels of new users that started as legitimate prescriptions. So now, it’s down to the people that would likely have tried opiates without being fed them by the medical industry. Just speculation though.