<p>Artificial radiation belts produced by high-altitude nuclear explosions consist of high-energy (mainly hundreds of keV to MeV) electrons. They are trapped by the Earth’s magnetic field, posing significant threats to spacecraft and human space activities. Understanding the mechanisms governing their evolution is crucial for both scientific and operational purposes. In this study, we propose a potential driver for azimuthal diffusion of these energetic electrons. Our guiding-center simulation results show that both the toroidal- and poloidal-mode ultra-low frequency waves can efficiently induce azimuthal diffusion of initially localized electrons, which occurs through nonlinear drift-resonance interactions. Electrons become trapped in resonance islands where their drift phase oscillates around a stable point in the wave frame. Bounce motion causes electrons to traverse different magnetic latitudes. This variation changes their local drift velocities and thus their effective <i>L</i>-shells, which alters the shapes of resonance islands and prevents electrons from returning to their initial drift phase. The cumulative phase shifts across electrons with different initial MLTs produce pronounced azimuthal diffusion. Electrons with smaller equatorial pitch angles diffuse more strongly due to their larger bounce ranges and enhanced phase accumulation. The resonance island transition process can further contribute to abrupt phase changes and thus amplify diffusion. These findings demonstrate that nonlinear drift resonance, bounce motion, and resonance island dynamics collectively govern azimuthal transport in artificial radiation belts. They provide a framework for modeling electron evolution under varying wave and particle parameters. And they also enhance our understanding of how wave-particle interactions influence the evolution of artificial radiation belts.</p>