Supersonic flow over a three-dimensional rectangular cavity with length-to-depth ratio of 2 is numerically studied by implicit large-eddy simulation to clarify the feedback-loop mechanism. A feedback-loop cycle is described and visualized with phase-averaged analysis of simulation results. Causality between the feedback acoustic wave and leading-edge shedding vortex is clearly demonstrated. Mach wave reflection at trailing edge is turned out to be the generation mechanism of feedback acoustic wave. It is convinced by investigating time-series instantaneous flowfields and auto-correlation coefficients of three simulation cases with different convective Mach number. Components of compression waves in supersonic cavity flows are summarized and their features are discussed. Proper orthogonal Decomposition (POD) in frequency domain is firstly employed to analyze wave propagations inside cavity. Results statistically show the propagation traces of notable compression waves inside cavity which are affected by high-speed recirculation flows.