The theoretical model is suggested which describes a new micromechanism of strengthening in the ultrafine-grained Al-Cu-Zr alloy subjected to severe plastic deformation. The departure point in this theoretical model is the assumption that the key role in the plastic deformation of high-pressure torsion processed ultrafine grained Al is played by extrinsic grain boundary dislocations (EGBDs) gliding along non-equilibrium grain boundaries and forming dislocation pile-ups at triple junctions of the grain boundaries. Within the model, nanoprecipitates of Al2Cu at grain boundaries act as obstacles for the slip of extrinsic grain-boundary dislocations (EGBDs) that leads to a significant increase in the strength of the Al-Cu-Zr alloy. The plastic deformation occurs through the emission of lattice dislocations from the pile-up of EGBDs pressed to a triple junction of grain boundaries. It is shown that the division of gliding EGBDs into separate pile-ups by nanoprecipitates can provide substantial additional hardening of the alloy. The proposed model is in good quantitative agreement with available experimental results.