Gibbo, it might well have had a lot to do with trying to extend your flight, although obviously it is important to ensure that the student is prepared for the manoeuvre and happy to try it before proceeding. It might also be that his natural instinct took over when he was given the opportunity of strong lift off the tow.
In all honesty I doubt that he was trying to frighten you or make you feel ill, it is counterproductive to the Club's interests as they are trying to attract new members from trial flights and anyone who is permitted to carry out these flights would be well aware of this. Having said that, many of those who are permitted to carry passengers for trial flights in gliders are not, in fact instructors but simply experienced solo pilots who have been given a passenger carrying rating.
At times, even well established power pilots feel disorientated in gliders. The constant steep thermal turns, coupled with a reclined seating position being contributory factors.