The aim of the present work is to contribute to a better understanding of the physical mechanism causing the low frequency oscillations of the Shock Wave Boundary-Layer Interaction (SWBLI) by analyzing data from highly resolved unsteady numerical simulations. Results show that the low frequency reflected shock-wave motion is mainly driven by the low frequency breathing of the separation bubble that comes from a modulation of the successive enlargements and shrinkages of the separated zone (medium frequency flapping of the shear-layer); the oscillations of the reflected shocks foot being in phase with the motion of the separation point.