Maybe not necessarily natural, but I think you underestimate how much radioactive material is out among the general public (i.e. outside of power plants and missile silos). Radiology as a medical field literally revolves around using radioactive materials for medical imaging, and there have indeed been incidents where regular people got their hands on some canister of the stuff (that didn't get properly disposed of), oohed and aahed as it glowed in the dark and they painted themselves with it, and a week later dozens of people are dead and hundreds injured from radiation sickness.