This is a computational fluid dynamics simulation of an airfoil that is oriented to look like a vertical stabilizer. After watching the video, it becomes clear that the tip of the stabilizer causes a counter-clockwise vortex (if looking at the trailing edge). According to this answer, vortices only form on vertical stabilizers if a control surface (the rudder) is deflected. In this simulation, the rudder appears to be completely in-line with the rest of the airfoil.
Furthermore, there also appears to be a slight clockwise rotation on the bottom of the stabilizer, where it would attach to the aircraft's tail. This seems even harder to explain, given the opposite rotation and angular velocity of the top one. I can only assume this is partially caused by the disturbances from what look like hinges farther up. Is this reasonable?
TL;DR: What causes the strong vortex on the tip of this vertical stabilizer?