Our society increasingly relies on spacecraft operations in the Earth's inner magnetosphere, particularly for communications. Long-duration high-intensity fluxes of relativistic electron are hazardous to spacecraft operational systems. Adverse effects of these energetic electrons on spacecraft has resulted in significant public interest and renewed efforts to advance our understanding and predictive capabilities of relativistic electron flux variations in the inner magnetosphere. The flux variations are especially dynamic during geomagnetically disturbed times. It is often observed that fluxes of relativistic electrons in the Earth's inner magnetosphere decrease by orders of magnitude, followed by a substantial enhancement of up to two orders of magnitude above the pre-storm levels. This work primarily focuses on the investigation of two physical processes for the relativistic electron flux variations: The fully-adiabatic effect and the delayed substorm injection mechanism. We simulate fully-adiabatic variations of electron fluxes for the special case of equatorially mirroring electrons using Rice magnetic field models and a quiet-time electron flux model. The storm-time electron fluxes can be obtained by fully-adiabatically evolving pre-storm fluxes using Liouville's theorem. Our study shows that the fully-adiabatic effect can cause a flux decrease of up to almost two orders of magnitude for Dst = -100 nT. We also simulate acceleration and injection of plasma sheet electrons during substorm dipolarization using a 3-D MHD field model. The test particle simulation shows that tens-of-keV plasma sheet electrons may be accelerated up to relativistic energies during a rapid substorm injection followed by a slow radial diffusion to the inner magnetosphere. Comparison with measurements shows that the mechanisms may contribute significantly to the observed flux variations.