After studying the wreckage and where all of the pieces landed, I think that I can completely determine what happened in the few moments before impact.
We lost visual sight of the rocket in the trees, and never saw the ejection charge fire. However, from the soot and black-powder remnants left on the rocket I can conclusively determine that the ejection charge definitely went off before the rocket split up. I believe what happened at this point was instead of ejecting the forward section from the interstage and deploying the flagging to slow its descent, it instead blew the rocket apart right before the interstage, keeping the forward part of the rocket and the interstage together. The flagging, sadly, was ripped to shreds by the shock and small pieces were scattered around the area. When exposed to the cold the flagging became much more brittle than I had expected. Once the rocket was split into two pieces, the aft piece of the rocket was still composed of three layers of cans. The middle layer made contact with a dead tree, puncturing a can and separating it from the other two pieces. I reasoned this from the lack of damage on the other two layers of the aft section, and the placement of all three layers. There was also an ugly looking dead tree that was in just the right place for it to all make sense.
Captions under full-sized images.