 Solution

For an elementary bimolecular forward step, $v=k_2[{\mathrm{R}}_1][{\mathrm{R}}_2]$. A liquid solution is much less compressible than a gas, so applying pressure changes its volume and reactant concentrations only modestly. With composition and temperature otherwise fixed, the observed substantial acceleration is therefore most naturally assigned to a change in the rate constant.

This is an approximation, not exact incompressibility: for precise pressure kinetics, correct the concentrations for compression and distinguish them from pressure-dependent activities.