Radioactive decay occurs when unstable isotopes have their nuclei broken due to atomic instability. When this occurs, the radioactive decaying atom with a large nucleus probably contains a high ratio of neutron protons.
To understand why an isotope disintegrates we need to focus on the atomic nucleus. The nucleus is known to be charged with positive particles (protons), and they are very close to each other. It is also true that particles with equal charges repel each other. Therefore, the proximity of protons causes them to repel each other in an attempt to take as much space as possible. As a result, the core breaks because it cannot handle these repellent charges.
