In the present work we explain the anomalous behavior of the attenuation of the longitudinal acoustic phonon in GaAs as a function of the phonon energy ω in the sub-THz domain. These attenuations along the [100] direction show a plateau between 0.6 and 1 GHz at low temperatures. We found an excellent agreement between measurements performed by some of us, and new ab initio calculations of third-order anharmonic processes. The formation of the plateau is explained by the competition between different phonon-phonon scattering processes as Herring's mechanism, which dominates at low frequencies, saturates and disappears. The plateau is shown to be determined by the phononic final-state phase-space available at a given temperature. We predict that a change of scattering mechanism should also show up in the attenuation of silicon around 1.2-1.7 THz, and argue that the attenuation plateau is a general feature of cubic semiconductors.