Your Video
Even though the resolution isn't super high, and the camera is a great distance from the plane, if I had to place a bet I would say pretty certainly this was tail flutter. If you look at the aircraft just before the even occurs, things like the exhaust stacks are reasonably sharp. After the even begins, virtually everything on the aircraft is more blurry.
Before the aircraft begins climbing, there is a short abbreviated roll left and right. It doesn't look like much here because of the distance; however, I suspect in the cockpit it was quite violent. If the flutter was related to the trim tab it could have become asymmetric (caused the elevators to become out of phase with each other with one moving up when the other one is moving down. At these speeds and with such a highly modified aircraft, who knows what that might have caused in the way of controllability problems.
If the pilot was seated as shown in the picture I posted earlier in the thread, I think he might have been out from the first roll left and right. Everything from there on was just physics and aerodynamics.