Diameter of the thermal vs loss of lift when banking for a right turning radius.
If the thermal is wide enough, any pattern could do. If it's too narrow, any attempt at flying a circle inside of it will become an exercise of keeping maybe just the tip of one wing inside the thermal.
I don't know, but surely it's to have more of the flight times in the warm centre of the updraft. Circles would miss the centre region for at least half the flight. Also a looped square would reduce the amount of flight when you're pitched, reducing strain?
I'd expect they might do a crossover in the middle to do turns to both sides (left wing down, right wing down); but perhaps flyers prefer one side for turns.
