I've been in this with 63 other airplanes. There are groups of trainers, fighters, bombers and cargos that work together. But... we're only human and there will always be one pilot out of synch or just "not get it".
Obviously very hard to know exactly what separation provisions had been planned, either at the briefing or directly from the Air Boss at the time - with your experience at that same venue, Bobby G, what would they have been trying to achieve, do you think? From the video it looked like three fighters at higher speed passing closer to the crowd, and the B17 and another larger machine spaced further out, moving slower?
The P63 comes in fast, looks like he's got his attention on the fighters ahead but has inadvertently set up a greater turn radius than them, which sadly puts him in the same place in the sky as the B17. As Two's In says, he's belly up and unlikely to have been able to see the B17 in the seconds leading up to the midair. Would there have been a requirement (briefing or direct via radio) for the fighters to have sighted the bombers and avoid them laterally, i.e. pass clear, closer to the crowd? (which would make a lot of sense with the differing speeds)
RIP fellow aviators.