Supersonic turbulent flow over a compression ramp is studied using wall-resolved large eddy simulation (LES) with a freestream Mach number of 2.95 and a Reynolds number (based on the incoming boundary-layer thickness δ0) of 63,560. The unsteady dynamics of the present shock-wave/turbulent boundary-layer interaction (STBLI) are investigated using dynamic mode decomposition (DMD), linear and nonlinear disambiguation optimization (LANDO), and global stability analysis (GSA). The results reveal three dynamically important modes with characteristic spanwise wavelengths of 2δ0, 3δ0, and 6δ0. By examining the connections between the unsteady motions and the revealed modes of the present flow, different mechanisms for the dynamics of STBLI flow are discussed, including Görtler instability, global instability, and the convective instability that is excited and maintained by dynamics of large-scale motions from the incoming turbulent boundary layer. The coexistence of different mechanisms is confirmed in the present study.