what does exactly make heavy nuclei unstable?

In a heavy nucleus the number of protons and neutrons make the size of the nucleus large. The strong nuclear force is a short range force, but the larger size of a heavy nucleus results in comparatively large separation of the nucleons. So, for two protons at the two ends of the diameter of the nucleus the Coulomb’s repulsive force will reduce strong attractive force. This makes the heavy nucleus unstable. In simple words, in a heavy nucleus the binding energy per nucleon is low as a result a small energy can easily split the nucleus.