We analyze the influence of a glass transition gradient near the particle surfaces on the nonlinear mechanical behavior of reinforced elastomers. We studied systems consisting of grafted silica particles dispersed in a cross-linked poly(ethyl acrylate) matrix. Both particle/matrix interfaces and dispersion states of the particles have been precisely characterized. On the basis of previous studies on the same systems evidencing a glass transition gradient around the particles, we show that the precocious nonlinear mechanical behavior of these filled systems is related to the strain-softening of the glassy polymer shell surrounding the particles surfaces. Comparing strain-softening and melting of the glassy polymer shell, we observe that the strain-softening is enhanced by strong local stress amplifications.