Nuclear energy comes from mass. Nothing causes it. It is like things dropping under gravity. There is a transformation of gravitational energy into kinetic energy.
Nuclear energy is the energy released when nuclear reactions take place.
In certain nuclear reactions, the sum of the masses of the reactants is more than the sum of the masses of the products. For more details, please consult some physics textbook at college level. The former is a spontaneous process, which however requires to build up to a chain reaction before large quantities may be extracted. Fusion requires extreme conditions of temperature and sufficient density. All present day nuclear reactors are fission reactors.