Doesn't a person sitting on the wing (even if it's just one of two in a biplane) massively disturb the airflow on that side? To my amateur eyes it looks like a sudden wing drop on the side with the wing-walker (on the wingtip, too), obviously with no chance of recovery at that height, not any technical problem.
And of course, first and foremost, it looks harrowing :-( At least it was apparently all over very quickly.