The answer is; Heavy nuclei split into lighter nuclei
Heavier isotopes of elements are the most unstable and decay quickly into lighter elements (mostly because they have high neutron to proton ratio). The decay involves the splitting of the heavier atom into a lighter atom and the release of neutron(s) particles and radiation energy. The released neutrons then hit another isotope and split it. This chain of reaction continues so far as the isotopes are close by. The time it takes for a radioactive element to decay into another element is called half-life.