The need for a better understanding of the low-frequency unsteadiness observed in shock wave/boundary layer interactions (SBLI) has driven research in this area for several decades. While numerous studies have been conducted on interactions with a turbulent boundary layer, in the context of transitional SBLI, research is still in its early stages and the low-frequency unsteadiness and the mechanisms underlying its origin remain poorly defined. In this study, large eddy simulations (LES) are performed in a \(M=1.7\) transitional shock reflection with separation. The objective is to examine any unsteadiness and the underlying mechanism. Beginning with a thorough assessment of stability theory, which suggests that transition in low supersonic compressible flows arises from the breakdown of oblique unstable boundary layer modes, the following question is posed. Do non-linear couplings between these oblique unstable modes and low-frequency unsteadiness emerge, and to what extent? To investigate quadratic couplings, high-order diagnostic is required. The results demonstrate that the unstable modes of the boundary layer interact non-linearly. High-frequency modes cascade non-linearly towards higher frequencies, initiating the turbulent cascade process, and towards lower frequencies. The low-frequency quadratic coupling with the flow characteristics at the separation point is responsible for the unsteadiness.