The causes can only be one of these:
1) Mechanical failure of something in the flight control system of the P-63. Since aircraft parts are subject to rigorous inspection, they don't just "break", so a failure would come down to human error during restoration or maintenance. Not a frequent occurrence, but it happens. I've worked with a few A&Ps I wouldn't let put gas in a lawnmower.
I know of a fatal King Air crash, shortly after takeoff, where an "A&P" with a bogus license failed to properly reconnect the ailerons during a 100-hour inspection.
2) A massive medical issue experienced by the P-63 pilot, causing him to lose consciousness and control of the aircraft. This has been known to happen.
3) Absolute incompetence on the part of the P-63 pilot, or unfamiliarity with the aircraft. Possible, but not likely.
4) Deliberate collision. Extremely unlikely.
5) B-17 being in the 'wrong place' according to planned maneuvers during the show, but it would still be the responsibility of the P-63 pilot to not fly into it.
It will be interesting to see the NHTSA/FAA findings from the investigation.