My headcanon is that it’s an interaction between cavitation from the object dragging through the fluid and recoil from the particles flinging off of the surface

My headcanon is that it’s an interaction between cavitation from the object dragging through the fluid and recoil from the particles flinging off of the surface. If you move fast enough then the momentum imparted by the ram surface onto the molecules are large enough to overcome the external pressure trying to fill the cavitation, so you get boundary layer separation.