Basically, Mr Hill commenced a looping manoeuvre at too low an altitude to complete it safely.
We all know that. The problem is nothing to do with ejector seats - it's to do with airline pilots turning up at air shows expecting skills that they once had - and practiced regularly (weekly in the season) to still be there.
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Or too slow a speed to achieve a top height. The point here is surely designing a display with enough margin in it to forgive the odd error or variation without disastrous consequences. An obvious point in hindsight.