Ahaaa Nuts.

It reminds me of my ex-father in-law who was doing acro over remote wet lands, two decades ago. The only witness was a pipe line worker who saw his plane in a dive; it did not pull out. Like this article, they thought may be he hit is head and was knocked out or passed out for some reason. They never really determined the cause. I have whacked my head a few times on the canopy flying, once near the hills approaching Reno. I still had the belt lose for en-route comfort and was not ready (belts tight) for landing yet; keep those belts tight.