What causes spontaneous radioactive decay?
There are different kinds of radioactive decay and different mechanisms. Unstable nuclei are relatively weakly bound with respect to their nucleon constituents, and easily fall apart into more-tightly-bound constituents. Typically the nucleus is in a “local minimum” of energy and needs a small kick to make it fall apart (see above question too). This kick often comes in the form of a “random fluctuation” of energy which causes a reconfiguration of the nucleus: for instance, sub-components of the nucleus can “tunnel” out. We’ll talk more about this sort of thing when we get into quantum mechanics in a bit.
