The goal of this study was to characterize the mechanism by which a swirl-stabilized gas turbine combustor spontaneously transitions from stable operation into a combustion mode with strong, self-excited thermoacoustic oscillation. This was accomplished by applying highbandwidth laser- and optical imaging techniques to acquire long-duration (4s) time-series measurements of a target flame undergoing frequent, spontaneous transitions between stable und thermoacoustically excited states. The target flame was a turbulent, swirl-stabilized ethylene-air flame operated at ϕ = 0.91, and 5 bars pressure. Stereo-PIV measurements, acquired at 9.3 kHz over periods of approximately 4 seconds were used to characterize the flow-field near the exit plane of the combustor. Acoustic measurements and OH*- chemiluminescence images were acquired synchronously, with OH* images acquired at every third cycle of the PIV measurement system. Wavelet-based analysis was employed to identify possible precursors to transition (to and from the stable state). One potential precursor was a 635 Hz oscillation that appeared ≈ 0.15s prior to transition. Analysis showed this oscillation to be thermoacoustic in nature and to strongly affect the outer shear layer of the flame, but the physical mechanism linking it to transition was unclear. In this study we show the physical mechanism associated with this precursor was a Helmholtz resonance that couples to the outer shear-layer of the reactant inflow, which leads to enhanced transport of hot reacting flow into the ORZ, destabilized the outer shear-layer and rendered the flame more susceptible to forcing by the (720 Hz) resonant acoustic mode of the combustor/pressure-vessel.