In the excavation of underground engineering, the internal energy stored in rock elements is suddenly released due to the effects of excavation unloading and stress concentration in the surrounding rock, resulting in the dynamic ejection failure of rock blocks. To reveal the energy evolution patterns and failure mechanisms associated with rock pillar bursts in underground structures (e.g., deep-buried tunnel pillar sections and intersecting tunnels) under high-stress triaxial unloading conditions, a self-developed true triaxial testing system was employed in this study. Given the mechanical characteristics of four-sided free-rock pillars, a series of simulation tests of pillar bursts were conducted at varying confining pressures. Red sandstone specimens measuring 200 mm × 100 mm × 100 mm were used at three confining pressure levels (10, 30, and 50 MPa). The stress path followed was “synchronous loading of triaxial initial stress－rapid bidirectional horizontal unloading－axial stress concentration”, designed to simulate the two-way excavation unloading and stress redistribution process in rock pillars. The failure process, debris ejection velocity, and macroscopic crack propagation were recorded using high-frame-rate cameras, particle image velocimetry. Under a combined first law of thermodynamics and true-triaxial energy calculation method, the accumulation, transformation, and release patterns of total, elastic, and dissipated energies were systematically analyzed, and the role of confining pressure in rockburst energy evolution was clarified. The results show that: (1) Under the triaxial horizontal rapid unloading path, the peak strength of rock specimens exhibits a significant confining pressure effect. The rate of postpeak stress drop increases significantly with higher confining pressure, and the brittle failure characteristics of rockbursts are significantly enhanced. (2) Specimens exhibit considerable plastic deformation and unloading dilation during the unloading stage, a mechanical response difficult to replicate in conventional triaxial tests. (3) A distinct competitive evolution mechanism exists between dissipated and elastic energies across different confining pressure levels. (4) The three types of energy at peak stress in rock specimens progressively increase with the rise in confining pressure, ranking as follows: total energy > dissipated energy > elastic energy.