BD-5 is a demanding plane to fly. Small wing, relatively low thrust from the jet, not much fuel capacity, and I've read it had some handling quirks. Not too much for someone with jet experience to handle, but it would sure bite a careless or inexperienced pilot fast.
I wonder what happened. Airshow flying can cause stress on the airframe that the original designer never anticipated, so it could have been anything from structural failure to simply biffing into the ground. Since the engine kept running after the crash, it sounds like he may have just lost it or bottomed out of a maneuver too low and whacked the ground a bit too hard. But that's just speculation.