IDK but one factor is most of the water filling the gap would come from underneath (it's at higher pressure), moving up to the surface. Its momentum carries it

IDK but one factor is most of the water filling the gap would come from underneath (it's at higher pressure), moving up to the surface. Its momentum carries it slightly too high, so it flows outward, then down and inward again, forming a vortex on either side. The surface looks smooth and the twin vortices woukd tend to persist.