Turbulent mixing generated by sloshing tidal motions along sloping ocean topography is key to explaining the upwelling of waters in the abyssal branch of the Meridional Overturning Circulation. The Boundary Layer Turbulence field campaign has provided direct evidence for strong upslope flows along a deep submarine canyon of the Rockall Trough. Turbulent overturning events spanning 200 m in the vertical were observed every tidal cycle, suggesting that the strong tidal flows in the canyon periodically undergo some form of instability. To determine what drives these turbulent overturns, this study combines observational analysis, numerical simulations, linear stability analysis, and approximate analytical solutions. The results show that the tidal flow is stable to both convective and steady shear instabilities but is destabilized by its oscillatory nature through a parametric instability. The findings suggest that turbulent mixing induced by oscillatory shear flow may be ubiquitous over sloping topography and play an important role in deep ocean mixing that supports the global overturning circulation.