Investigating the primary drive mechanisms in underground hydrogen storage (UHS) is crucial for optimizing hydrogen recovery from geological formations. In this study, we used a 3D heterogeneous geological model to assess the dynamics of hydrogen storage in an aquifer under various hydrodynamic and geological conditions, using material balance to quantify the contribution of different production mechanisms including gas expansion, water influx, exsolution, and the compressibility of rock and water. Our findings indicate that gas expansion and rock compressibility are the predominant mechanisms driving hydrogen recovery for closed boundary conditions with minimal aquifer support, whereas water influx dominates when there is substantial aquifer connectivity. Exsolution as a driving force has a negligible impact on hydrogen recovery, simplifying the modelling of storage dynamics. Furthermore, cushion gas injection was found to notably improve gas expansion under open boundary conditions, optimizing hydrogen recovery during the initial production phases. The study underscores the influence of uncertain parameters and processes on hydrogen storage efficiency, offering critical insights for the design of UHS and identifying key reservoir and fluid properties necessary for robust management.