What are enthalpy changes of physical, chemical and nuclear changes due to

Would it just be the breaking and forming of bonds and energy released/absorbed?

That would be the reason for enthalpy changes of chemical reactions.

For physical changes, it would be partly the enthalpy of a phase change. For example, it takes 80 calories input per g to melt ice to water or given off in freezing water to ice.

For a nuclear reaction, it is the difference between the binding energy of the nucleons of parent and daughter nuclei.