We have previously described a numerical model for carrier diffusion and nonlinear quenching in the track of an electron in a scintillator. Significant inequality of electron and hole mobilities predicts a characteristic “hump” in the light yield vs gamma energy, whereas low mobility of either or both carriers accentuates the universal roll-off due to nonlinear quenching at low gamma energy (high dE/dx). The material parameter basis of the two major trends in nonproportionality of scintillators can be related to the effective diffusion coefficient of excitations and the difference of electron and hole mobilities, respectively. Activator concentration, type of activator, and effect of transport anisotropy are associated with minor trends. The predicted trends are qualitatively consistent with empirical measures of nonproportionality including electron yield curves.