Silica-filled polydimethylsiloxane networks are submitted to successive stretching cycles, in order to get the stabilized stretching curve, at variable temperature. This study explains the peculiar temperature dependence of the first stretching curve of filled rubbers, and highlights the molecular origin of the stress-softening phenomenon, known as Mullins effect. Thanks to the comparison between the strain dependence of stress and the molecular orientation, this effect is attributed to the detachment from the filler surface or slippage on the filler surface, of chains having reached their limit of extensibility. Moreover, by taking advantage of Atomic Force Microscopy observations on stretched samples, the Mullins effect is shown to take place mainly in regions of high local concentration of silica. The experimental results are also compared to Bueche's model for the Mullins effect.