NTSB Factual Report

The pilot was landing on a gravel airstrip, and the airplane bounced during touchdown. When it touched down again, the pilot began to apply the brakes, and as expected, the tail of the airplane began to rise slightly. The rear seat passenger did not realize that this condition was normal, and braced his feet against the rudder pedals, which applied increased brake pressure. The airplane nosed over resulting in substantial damage to the rudder, vertical stabilizer, and wing lift struts.

NTSB Probable Cause

The passenger's inadvertent application of the brake pedals during landing, which resulted in a noseover.