A staged injector developed by JAXA and fueled with kerosene is studied in a high-pressure combustion experiment. With a stable pilot fuel flow rate, the fuel flow rate in the main stage is progressively increased. A high-speed OH-PLIF system is used to record the flame motion at 10,000 fps. In the beginning of the recording, POD modes shows that the flame behavior is dominated by relatively low-frequency rotation due to the swirling motion of the flow. These rotational motions then coexist with a thermo-acoustic instability around 475 Hz which increases the amplitude of the pressure fluctuations inside the chamber. DMD analyses indicate that this instability is associated with a widening of the flame occurring when the pressure fluctuations are the highest, giving the instability a positive feedback. The instability frequency then abruptly switches to 500 Hz while retaining the same driving mechanisms. Potential candidates for this frequency change are proposed.