Using ab initio multiconfigurational potential-energy surfaces pertinent to the reaction Zn(P-3) + H-2-->ZnH((2)Sigma) + H and local surface tessellation and interpolation methods developed earlier, we carried out classical trajectory simulations of the title reactions, running large ensembles of trajectories with initial conditions representative of full-collision experiments. Then, using binning techniques, the ZnH/ZnD vibrational and rotational populations have been extracted. Our simulation results duplicate certain unexpected findings noted in experiments of Breckenridge and Wang [Chem. Phys. Lett. 123, 17 (1986); J. Chem. Phys. 87, 2630 (1987)]. Specifically, it is observed that the rotational profiles found for ZnH produced from H-2 or from HD are nearly identical, as are the ZnD populations obtained in reactions with D-2 or HD. By interrogating the progress of reactive trajectories, we have been able to identify the origin of (much of) the rotational angular momentum in the ZnH/ZnD products, which then allows us to put forth a physical model which, we believe, explains the unexpected ZnH/ZnD isotope effects in the product rotational profiles.