With the rapid deployment of flexible high-voltage direct-current (HVDC) systems, high-power press-pack insulated-gate bipolar transistors (PP-IGBTs) have become critical components. However, field data indicate premature overcurrent failures in PP-IGBTs under modular multilevel converter (MMC) operations. This study systematically investigates the short-term overcurrent failure mechanisms of PP-IGBTs under MMC-specific operational stresses, distinguishing them from conventional short-circuit or avalanche modes. A power cycling test platform emulating MMC submodule conditions was developed to capture the evolution of terminal characteristics. Furthermore, a double-pulse test platform was established to reproduce failure locations under service-relevant stresses. The study reveals a distinctive failure mode characterized by pronounced gate leakage and the eventual loss of normal gate control, leading to functional open-circuit failure—a phenomenon that has been rarely reported in the existing MMC literature. The experimental results indicate a decreasing trend in short-term overcurrent withstand capability with increasing clamping force and reveal a preliminary relationship between peak current and measured emitter-side temperature at failure. These findings provide insights into the failure mechanisms of PP-IGBTs and offer practical guidance for device selection, converter design, and reliability assessment.