The precise moment of radioactive decay is not a cause. It's an outcome. The cause lies in the underlying physics. Decay is an event that is driven by particle interacitons(force, dynamics...) and the uncertainty principle. The important forces can be IDed for any particular particle decay. It's the force and dynamics of the interactions that detemine the particle, or State lifetimes and widths and are the identified cause.
So you are saying that for two atoms, one of which decays in a given interval and one of which does not, there is an intrinsic difference between them at the start of the observation?