Gas turbines featuring sequentially staged combustion systems offer excellent performance in terms of fuel flexibility, part load performance and combined-cycle efficiency. These reheat combustion systems are therefore a key technology for meeting fluctuating power demand in energy infrastructures with increasing proportions of volatile renewable energy sources. To allow the high operational flexibility required to operate in this role, it is essential that the impact of thermoacoustic instabilities is minimized at all engine load conditions. In this case, high-frequency thermoacoustic instabilities in the second “reheat” combustion stage are investigated. Reheat flames are stabilized by both auto-ignition and propagation and, as a result, additional thermoacoustic driving mechanisms are present compared with more conventional swirl-stabilized combustors. Two self-excited thermoacoustic modes have been observed in a 1 MW reheat test rig at atmospheric pressure, one which exhibits limit-cycle behavior while the other is only intermittently unstable. The underlying driving mechanisms for each individual mode have been investigated previously and, in this paper, the two modes are directly compared to understand why these instabilities are each associated with different driving phenomena. It is shown that, due to the different flame regimes present in the reheat combustor, the potential for flame-acoustic coupling is highly dependent on the thermoacoustic mode shape. Different interactions between the flame and acoustics are possible depending on the orientation of the acoustic pressure nodes and antinodes relative to the auto-ignition- and propagation-stabilized flame regions, with the strongest coupling occurring when an antinode is located close to the auto-ignition zone. This provides insight into the significance of the different driving mechanisms and contributes to the ongoing development of models to allow prediction and mitigation of thermoacoustic instabilities in reheat combustion systems, which are crucial for reliable combustor designs in the future.