They arise from full nuclear shells of either protons or of neutrons. The emission of gamma rays, which are very short wavelength electromagnetic radiation, like x-rays, are regarded as resulting from either a proton, or a neutron, dropping from an excited energy level to a vacant lower one. The analogy here is in atomic emission spectra where light is emitted when an electron drops from an excited energy level. The electron analogy here is the phenomenon of phosphorescence in which can electron must reverse its spin in order to occupy a vacant lower energy level, a process which retards the emission of light. By convention the isomer corresponding to the upper energy state is indicated by an M after the mass number, to indicate that it is the metastable species.
Nuclear isomers are very interesting nuclei and possess many unique properties.
They tend to de-excite. In some instances, where the lower energy, or ground state, is itself radioactive, the metastable state and the ground state will behave like two different isotopes and have different half lives. They can even undergo different modes of decay. This is the case . Gamma emissions from isomers should always be of low energy.