Related to me by someone connected to the airshow performance:
The Airboss had briefed the fighters to stay high and bombers to stay low, with the option for the fighters to share the bomber altitude block if everything looked clear.
Believing that it was, and that the bombers were clear of the runway centreline, Airboss cleared fighters to come down into the bomber block.
P-63 pilot did not expect traffic on the runway centreline and was looking left, into the turn, when the collision occurred.