Enhanced mechanical softening accompanying nanoparticles reinforcement of rubber is an important source of energy dissipation and heat buildup of industrially engineered elastomers. Its mechanism previously assigned to damages in the filler network, rubber-filler interface and rubber phase remains controversial in more than 70 years. Through investigating the typical Payne effect of styrene-butadiene rubber gum and its vulcanizates as well as silica filled compounds and vulcanizates and the Mullins effect of unfilled and filled vulcanizates, we herein evidence that the filler-promoted softening of the rubber phase softens the filled elastomer nanocomposites. Especially we show that the Mullins effect is relevantly involved in the disentanglement/re-entanglement of dangling chains superposed on the entropically elastic network of the rubber phase. This paper clarifies that the mechanism of nonlinear mechanical softening for filled rubber compounds and vulcanizates should be rooted in macromolecular chains in the entanglement network (gum and filled compounds) or long dangling chain in non-ideally crosslinked network (vulcanized gum and filled vulcanizates), rather than damages involved in the "filler network" or filler-rubber interface. This suggests that adjusting the nonideally crosslinked network structure of viscoelastic rubber matrix should be able to optimize the use performance of the rubber nanocomposite products.