Two mechanisms for particle production, direct and spectator ones, are analyzed on the basis of the recently proposed QCD-based parton model for cumulative phenomena in interactions involving nuclei. It is shown that, owing to final-state interactions, the leading terms of the direct-mechanism contribution are canceled, so that the spectator mechanism is the dominant one. This leads to a smaller slope of the rates of cumulative particle production in relation to the slope of the nuclear structure function in the cumulative region x > 1. This result agrees with recent experimental data.