As mustafagander has said, the tailplane is an inverted wing.
For stability the centre of mass of the plane is ahead of the centre of lift of the main wing. Therefore the tailplane actually has to be pushing down to keep the plane level - in regular flight it would be providing a small downward force. Pushing down = higher pressure on the top than underneath = inverted vortices compared to a normal wing.
In the flare, it's pushing down more than usual - because of the low airspeed (more deflection needed for the same effect), because the main wing centre of lift has moved back (due to flaps being extended), and obviously because the pilot is trying to maintain a relatively high pitch attitude.