Someone earlier in this thread mentioned accidental take off.
If you look at the design of the aircraft, you can clearly see that the main gear are way aft of the center of lift.
Because of that configuration, the excessive downward force required by the elevator to cause the plane to raise the nose with the pivot point being the (too far aft) main gear would guarantee pitch-up once the wing generated lift and the "pivot point" changed from the main gear to the center of lift.
The fact that nobody was filming the crowning achievement of his first flight leads me to believe he did not intend to take off.
I feel he was doing high speed taxis with a bit of elevator deflection to lighten or maybe even raise nose so he could get a feel of the elevator authority but the wing generated lift, the "pivot point" swapped from the gear to the wing greatly increasing the authority of the elevator to raise the nose further, it pitched up sharply, and the wings blanked the airflow to the elevator which stuck him nose high.