The July 2025 Thailand-Cambodia border conflict marked the first use of air power in the region since 1988, standing in stark contrast to the 2008-2011 clashes over the same disputed territory, which remained a managed conflict despite similar nationalist pressures. To explain this divergence, this study develops an integrated analytical framework by adapting Feaver's principal-agent theory and Fearon's audience costs theory to Thailand's distinctive political context. The analysis reveals that the uncontrolled escalation in 2025 resulted from the convergence of two causal pathways: a "dual-principal structure" institutionalized by the 2017 Constitution, and "externally imposed audience costs" triggered by Hun Sen's strategic leak of a private phone call. Under this dual-principal structure, the Thai military aligned with the de facto principal―the monarchy-military network and the Constitutional Court―rather than the elected government, enabling commanders to defy ceasefire orders without consequence. Meanwhile, the Constitutional Court removed the civilian leader who sought de-escalation, producing a "reversal of punishment" that facilitated autonomous military escalation. By contrast, the 2008-2011 conflict remained controlled due to preference convergence between the Abhisit government and the military. This study demonstrates that in hybrid regimes such as Thailand, the formal appearance of civilian control does not guarantee effective control over the armed forces.