To design and optimize the next generation of lean-premixed gas turbine combustors, understanding the combustion instability characteristics at high-intensity turbulent flows is necessary. In lean premixed turbulent combustion, the onset of instability is potentially influenced by various geometric and flow parameters. Whereas, the role of inlet flow turbulence on the stability characteristics of a combustor still needs better understanding and critical insight into the flow-flame interactions that drive combustor dynamics and flame stabilization. Time-resolved investigation of flow-field and flame structure at elevated pressure yields a novel data-set. It reveals an important mechanism and phenomenon for studying the dynamics of turbulent combustion. The present work is focused on the effect of inlet turbulence intensity on the thermo-acoustic combustion instabilities and thermo-acoustically induced flame flashbacks in the backward-facing step combustor. The turbulence generator was placed upstream of the flame holder and used to vary the turbulence levels. The analysis of multi-variate high-speed data acquisition and processing (viz. unsteady pressure, flame-front extraction, and velocity field utilizing simultaneous PLIF/PIV) reveals the role of low-frequency high amplitude acoustics in modulating the flame surface and reactive flow-field. It is seen that high amplitude oscillations are sustained by two mechanisms 1. Modulation of the flame by coherent structures shedding at the step and 2. The bulk flame motion in-and-out at the edge of the step. It is seen that the flow reversal process reinforces the coherent unsteadiness in the system, thereby increasing the propensity of the mixture to be ignited more upstream with every cycle. Ultimately leads to the flame flashing back to the point of premixing. This work thus addresses and reforms the understanding of reacting turbulent flow dynamics under acoustic excitations, particularly the influence of acoustic wave amplitudes under significant turbulence intensities on the flow and flame dynamics.