The widespread adoption of high-energy-density all-solid-state batteries (ASSBs) for electric vehicles, aviation, defense, and other safety-critical applications requires robust tolerance against accidental mechanical abuse. Despite growing attention on thermal instabilities in ASSBs, their response to mechanical abuse remains largely unexplored. Here, we present a comprehensive investigation of mechanical abuse in industry-relevant ASSB pouch cells comprising dry-processed LiNi0.8Mn0.1Co0.1O2 (NMC811) composite cathodes, slurry-processed Li6PS5Cl (LPSCl) separators, and high-capacity Si and Li metal anodes. Controlled indentation experiments on Si-based ASSBs reveal fundamentally different failure behavior from conventional lithium-ion batteries, characterized by extended plastic deformation, delayed internal short-circuit (ISC) initiation, and soft-short behavior under zero stack pressure. X-ray computed tomography reveals a shear-dominated fracture mechanism triggering an ISC through crack-induced layer sliding and cathode-anode contact. While thicker separators can moderately improve mechanical abuse tolerance, increasing stack pressure promotes low-resistance ISC pathways that significantly intensify failure severity, identifying stack pressure as a critical operating parameter governing ASSB safety under mechanical abuse. Experimentally informed thermo-electrochemical modeling further highlights that high state-of-charge (>50%) and elevated stack pressures (5 MPa) poses a critical bottleneck during mechanical abuse, driving ASSBs into thermal runaway scenarios. Finally, we reveal that, unlike Si anode, Li metal-based ASSBs exhibit a fundamentally different failure mechanism, wherein lithium creep and plastic extrusion through mechanically damaged regions lead to catastrophic ISC even under zero-stack-pressure condition. Collectively, this work establishes the mechanistic framework linking mechanical failure, ISC formation, and thermal instability in practical ASSBs, while providing actionable design and operational guidelines for safer, more mechanically resilient ASSBs.