I agree with Arthur, that the seat could have slid back.
That seat rail, on the L-19 was always fairly flimsy to begin with, if it is similar to ones that I worked on while I was stationed in Germany. Then again, there was the powerful turboprop engine that will rock you back in your seat on takeoff. Hopefully, the seating wasn’t too badly burned so that they can actually find out what happened. The fire department was slow to respond as far as I could see, twisting and turning to get to the scene of the accident. RIP to the pilot.