More specifically, given two atoms of the same isotope, what causes one to decay before the other?
It must be the work of the Intelligent Designer. We can know no more than that.
Here's one for the designer: Given a single atom with a half-life of 10 minutes, what is the probability that it will decay within the next ten minutes? Assuming it does not decay in the next ten minutes, what is the probability it will decay in the following ten minutes?
Lather, rinse, repeat.