Who knows? The point is that he was able to. It could have been a simple mistake. Although he was a test pilot, and he could have had a reason that made sense to him. Whatever, the hardware allowed him to do something that turned out to be fatally dangerous, presumably without giving him any indication that it really wasn't a good idea.
(There's a famous story of a Harrier jump jet test pilot asking "What happens if I vector the thrust nozzles during forward flight?" To which the answer was "We don't know - no-one's ever tried it." So he did. And nothing went wrong, and the Harrier's then-unique VIFFing manoeuvers were born. But there was always an outside chance that it would turn out to be a really bad idea, and that things could have gone horribly wrong, perhaps even killing the pilot. In which case we'd be asking what he was doing, doing something so stupid? Answer - doing his job.)
In my book, that's not pilot error. That's either a design failure, or a training failure, or a test pilot pushing the envelope beyond its limits.