The reason most atoms undergo nuclear decay is because

The reason that most atoms undergo nuclear decay is because the nucleus is unstable and breaks apart. The nucleus of the atom contains protons and neutrons. Since protons are positively charged, they should repel each other and consequently break apart the nucleus. But this does not always happen because the strong nuclear force between the particles is strong enough to keep them together and the neutrons present serves as a buffer between the protons. The tendency for the nucleus to break apart depends on the relative numbers of neutrons and protons. In large nuclei the strong nuclear force becomes less effective since it does not operate over larger distances. This causes the larger nuclei to break apart more easily.

The reason most atoms undergo nuclear decay is to attain stability. Most elements with 84 or more electrons are unstable, so the nucleus of the elements decompose and undergo nuclear decay forming new elements with lower number of electrons.