Someone said :
The scary part is that he had a running chain saw in his hand.
That could have done some serious damage.
Watching it again, you see how the underside of the branch is still attached to the tree and as the branch starts to give to gravity it has to tear away and break off when th far end hits the ground.
An experienced woodsman would have made an undercut on the branch before preceding to cut through from the top, thus preventing such an accident. Tsk tsk.
I am picturing you with your big axe, being the local lumberjack on brokeback hill!