Not all observed phenomena have causes. Radioactive decay is uncaused.
In the sense that no preceeding event determines the time of decay. (Aristotle's effecient cause.) One could say the material cause is: "It is the nature of radioactive atoms to decay at a random time, etc. (time-energy complementarity...)."
Ok, maybe I'm out of date here. I thought radioactive decay was a result of the structure of the relevant isotope?
Of course I learned this back when there were all these little planetary systems called hydrogen, helium etc..
In addition, chemistry was almost as opaque to me as calc.