We perform an electromagnetic particle simulation of triggered emissions in a uniform magnetic field for understanding of nonlinear wave-particle interaction in the vicinity of the magnetic equator. A finite length of a whistler-mode triggering wave packet with a constant frequency is injected by oscillating an external current at the equator. We find that the first subpacket of rising-tone triggered emissions is generated after termination of the injection of the triggering wave in the homogeneous magnetic field. By analyzing resonant currents and resonant electron dynamics in the simulation, we find that the formation of an electron hole in a velocity phase space forms resonant currents, and the currents cause wave amplification and frequency increase. As the very initial stage of the generation process, phase-bunching occurs at the wavefront of the triggering wave. The phase-bunching is caused by the rotation of electrons in the velocity phase space because of the gradient of the distribution function in the parallel velocity. The phase-bunched untrapped electrons are scattered to the loss cone giving energy to the electromagnetic waves, while the electrons in the low density region are trapped by the wave potential, forming an electron hole. The time scale of the initial formation process of the electron hole is related to the duration time of the triggering wave necessary for generation of triggered emissions. The duration time is determined by the interaction time. For the generation of triggered emissions, the interaction time is more than 1/4 of the nonlinear trapping period in the present simulation.