A demonstration of rotor - in this case behind a building - but the same occurs behind the sharp ridges we fly. Lord's \ Rushup is the obvious example, but Treak has a rotor-forming ridge that also affects top-landing behind it. Occurs behind trees too - behind the plantation on the way from Lord's lower to upper, for example.
Notice how at times, the turbulence extends well above the height of the obstacle.
Link should skip to 7.20 where the demo starts