BAFC operate PA28s, so presumably an instructional flight. Powered and Gliding takeoffs are done concurrently on parallel runways.
Did the glider actually come off tow? Or just reposition itself? IIRC on an easterly takeoff the glider and tug turn E/SE immediately crossing the motorway, so for an aircraft to go UNDER the tug/glider combo (which will hardly be climbing fast) something strange must be going on
A glider coming off tow there would not be fun for the glider pilot either!