This paper considers an electric power grid with multiple synchronous machines exhibiting coupled swing dynamics and aims at developing a hierarchical diagnosis of loss of transient stability of the machines, which we term as swing instabilities. We introduce a standard energy function for the power grid and decompose it into individual energy functions of a collective system of the grid, of the motions relative to the collective system, and of the infinite bus. This decomposition enables us to extract distinctive behaviors embedded in the swing instabilities in terms of energy. We numerically apply the individual energy functions in the proposed decomposition to analysis of swing instabilities in the IEEE New England 39-bus test system. We then propose a hierarchical diagnosis of the swing instabilities based on the decomposition and discuss its validity by approximating the individual energy functions along solutions representing the grid's instabilities.