I believe that one of the primary causes is the overprescription of opiods by doctors in the USA, who were pressured by a patients (a generation of consumerism and instant gratification who expect doctors to provide instant treatment/remedy) on one side, and drug companies (who saw prescription painkillers as a sudden new cash cow) on the other. An article I've recently read in the Atlantic provides a perspective on this subject:
https://www.theatlantic.com/magazine/archive/2019/05/opioid-...