A new mechanism is proposed to interpret the phenomena observed in the liquid fluidization of the binary particle mixture. Two main factors, the bulk density and voidage fluctuation, are considered to determine the position of the layer of particles in the bed. The new model successfully explains the layer inversion phenomenon and mixing-segregation equilibrium, which is the result of the above two factors acting together. The model also proves that bulk density is a simple and reliable method to predict the inversion velocity by comparison with the experimental results reported in the literature.