In a liquid-fueled two-stage gas turbine model combustor, a bistable point where two different flame states can exist is studied in non-reacting and reacting conditions with the help of fast spray-PIV and chemiluminescence imaging. The first flame, named sV, is stabilized thanks to an internal reaction zone that modifies the air flow compared to non-reacting conditions. The second state, associated with a lifted flame, exhibits a strong thermo-acoustic instability linked to the quarter-wave mode of the chamber and piloted by a varying liquid-fuel supply to the flame. Using well-controlled air flow rate modulations, the switch from the quiet sV state to the lifted one is studied in a time-resolved fashion. The analysis shows that the sensitivity of the internal reaction zone of the sV state to perturbations is responsible for the change in flame shape which then triggers the onset of the thermo-acoustic instability.