The critical cluster is normally made up of the generators near the fault location because their initial accelerations are in inverse proportion to the electrical distance between the units and the fault location. However， simulations on the national interconnected power grid show that remote units can also become the critical ones in some cases. Furthermore， it is also found that variation of the loads near the disturbance can not only change the critical cluster or the instability swing， but also can change both aspects simultaneously. Mechanisms are unfolded using EEAC stability theory. Based on theoretical analysis and simulation results， it has been verified that control effects are opposite not only between the critical cluster and the remaining cluster， but also between two neighboring swings if the same kind of control actions， such as generator tripping， are adopted. Further studies prove that the bifurcation on unstable modes may take place when the value of equivalent mechanical power is close to that of the aperiodic component of the post-fault electrical power.