Yeah I thought I saw a lanyard go down with the piece, maybe he roped into a fairly spindly tree, him bouncing at the end may be just the dynamic loading of his line?
I agree with boreality, start at the tip. It even looked like his climbing line orientation would have put him right over the crown to begin with, which would have made maneuvering much easier and could possibly avoid lanyarding in... even though its frowned upon to not lanyard with using a chainsaw, I believe there is certain instances where being lanyarded in might put you in more danger (tree's don't always give us the best cards in the deck to work with).
He had that big hunk of section that dropped secured with rigging rope too which makes you think about how big of a piece he took, what rope he was using, rigging point, tie-off? Theres a lot of variables that we can't really see which could explain how it happened, and how it could have been done differently.