Strong light-matter interaction in cavity environments has emerged as a promising and general approach to control chemical reactions in a non-intrusive manner. The underlying mechanism that distinguishes between steering, accelerating, or decelerating a chemical reaction has, however, remained thus far largely unclear, hampering progress in this frontier area of research. In this work, we leverage a combination of first-principles techniques, foremost quantum-electrodynamical density functional theory, applied to the recent experimental realization by Thomas et al. [1] to unveil the microscopic mechanism behind the experimentally observed reduced reaction-rate under resonant vibrational strong light-matter coupling. We find that the cavity mode functions as a mediator between different vibrational eigenmodes, transferring vibrational excitation and anharmonicity, correlating vibrations, and ultimately strengthening the chemical bond of interest. Importantly, the resonant feature observed in experiment, theoretically elusive so far, naturally arises in our investigations. Our theoretical predictions in polaritonic chemistry shine new light on cavity induced mechanisms, providing a crucial control strategy in state-of-the-art photocatalysis and energy conversion, pointing the way towards generalized quantum optical control of chemical systems.