We have ruled out all ordinary causes - those that obey the rules of standard logic, that either do or do not exist at any given moment, for example - for radioactive decay.We have read that radioactive decay is random, probably it is not. It is just that we are not able to decipher the trigger event. As I said there is nothing like True Randomness. If there cannot be true determinism then there cannot be true Randomness.
There is no evidence whatsoever for any "trigger event", and no one has any idea what it would look like, how it could possibly work or even exist.