What is the main Reason for instability of the nucleus?

Nuclear instability - a phenomenon concluded from empirical evidence. A model to explain this is based on the idea of nuclear shells. The number of protons and neutron in the nucleus. The nucleons fill up inside the nucleus in shells, each shell has a specific allotment of nucleons. incomplete shell gives rise to instability. The number of nucleons also affect the stability and such numbers are called 'magic numbers', 2, 8, 20, 28, 50, 82, 126. There are nuclides which have double the magic number, called 'double magic'. Magic and Double Magic numbers are charecterised by higher binding energies per nucleon for nuclides. The mechanism of stability may be calculated by using schroedinger's equation if the nuclear potential is known. in my opinion the instability and reason for randomness of decay may be due to vacuum fluctuations, connected to ideas from perturbation theory and definitely linked quantum field theory - any concrete mathematical evidence to support this is beyond me.