High amplitude combustion instabilities are a destructive and increasingly pervasive problem in gas turbine combustors. Although much research has focused on measuring the characteristics of these instabilities, there are still many remaining questions about the fluid-mechanic mechanisms that drive the flame oscillations. In particular, a variety of complex disturbance mechanisms arise during velocity-coupled instabilities excited by transverse acoustic modes. The resulting disturbance field has two components - the acoustic velocity fluctuation from both the incident transverse acoustic field and the excited longitudinal field near the nozzle, and the vortical velocity fluctuations arising from acoustic excitation of hydrodynamic instabilities in the flow. In this research, we look at the relative contribution of these two components as the amplitude of transverse excitation increases for a swirling flow and swirl-stabilized flame in a transverse forcing combustor that mimics the geometry of an annular combustor. Proper orthogonal decomposition is tested as a methodology for decomposing the velocity disturbance field and is used to understand the relative contributions of these two disturbance mechanisms.