I believe that according to QFT, nuclear decay events are attributed to the same thing that "causes" spontaneous emission of radiation from excited quantum states, namely, interaction of the metastable quantum system with a vacuum fluctuation (or virtual photon, or spaghetti monster tears, or whatever name you want to give to the hypothetical phenomenon). Some sort of interaction is required within the framework of quantum theory for excited molecular or atomic eigenstates to decay, because they are *eigenstates*, and thus their probability density is conserved.
So, the question now is, are vacuum fluctuations (or whatever) truly random? I don't know enough about QFT or quantum cosmology to even approach answering that question. Personally, I have a strong predilection to believe that they are in fact random, but it's just a gut feeling at this point.