It's radon washout, it's natural. Thunderstorms, in particular, can concentrate natural radon gas making rain temporarily highly radioactive. Naturally radioactive rain may cause 2% of skin cancer cases, it can even set off alarms at nuclear plants. It decays quickly, the half-life of Rn (and Bi & Pb daughter isotopes) is under an hour.

Ref.:

"ATMOSPHERIC WASHOUT OF RADON SHORT-LIVED DAUGHTERS: IMPLICATIONS FOR THE RADIATION DOSE TO THE SKIN"

"Intense gamma radiation from radon progeny accreted in/on rain during and following thunderstorms"