Very sad day . I have been ATC controlling a few airshows in a past life, now organizing some and flying in them in small SEP aircraft , not high-performance warbirds, but the routine and procedures are the same for as long as I can remember (+/- 50 years ) I have also been in the TWR on OSH during the show a few times and it is basically the same as we do in Europe.
ATC is not involved in an airshow during the show itself, just clearing the airspace., and watching out for eventual intruders. During the show itself the responsibility is with a show coordinator, also called Director, or Air Boss in the US. There is an extremely strict time planning and long briefings beforehand. During the performance the frequency is clear left for coordination between the aircraft., Responsibility for separation is strictly with the pilots performing themselves.
During my experience the most ckock-ups I have witnessed in multi aircraft performances where nearly always of 3 kinds, : either an aircraft is too late and speeds up to catch and try to re-enter a formation at high speed from an unusual angle, or when a replacement pilot is taking over at last minute with little pre-show training time , ,. the last one is when one of the aircraft is not where it was supposed to be at the right time . Sometimes a combination of those.
I am not saying this is what happened here, but the speed at which the P-63 tries to rejoin the formation makes me think it is again one of those. .
The investigation will (perhaps) infirm or confirm which one it is but. flying a demo in an airshow always carries a much higher risk..
Again, a very sad. day for the airshow community,