Observations indicate that the plasma sheet boundary layer (PSBL) becomes stressed during active times resulting in enhanced plasma flows, intense wave activity, and its minimum spatial width becoming on the order of the ion Larmor radius. We present simulations showing that the Electron-Ion-Hybrid (EIH) instability plays a key role during the nonlinear relaxation of such a stressed PSBL. The EIH is a long wavelength mode (kyLE~1, where ky is the perpendicular wavelength and LE the gradient scale size), its spectrum is broadband (extending from below the lower hybrid frequency to the electron plasma frequency), and its onset can generate substantial perpendicular ion acceleration and anomalous viscosity leading to particle mixing across the PSBL.