My best guess based on the video is that the P-63 pilot had been high and out of position in formation, and tried to get back in position using both geometry an

My best guess based on the video is that the P-63 pilot had been high and out of position in formation, and tried to get back in position using both geometry and speed. He may have recognized that he had too much closure and wasn't in position to safely underrun the B-17, and tried to bank and pull to get safe lateral separation, inadvertently putting himself into a blind lead turn where he wouldn't be able to see the B-17 through his wing or fuselage. The blind lead turn is an incredibly dangerous maneuver in formation flight--typically prohibited in military air combat training--for obvious reasons.