What is the key reason behind radioactive decay?

Protons are positively charged and a small collection of protons should repel each other. The presence of (neutral) neutrons permits a nucleus to contain many protons that do not repel each other. Many elements contain different numbers of neutrons but the same number of protons and are called isotopes (meaning same place on the periodic table of elements). However if there are too many neutrons (either naturally of by induced radiation by bombardment of neutrons) the nucleus may be unstable and will decay to a more stable state often in a chain of decay products.