People love to talk about cosmic rays, but in the business (of safety-related systems) it is well-known that most causes of single event upsets (SEU)/soft errors are the microchips (or their packaging) themselves.

"In the terrestrial environment, the key radiations of concern are alpha particles emitted by trace impurities in the chip materials themselves"

(https://www.ti.com/support-quality/faqs/soft-error-rate-faqs...)