I had that happen in my backyard once. Was a much bigger piece of wood. Made a hell of a racket and lit up the entire house at 3am. It was raining like crazy or else the falling fireball would have set the back 40 on fire.
It isn't so much the wood conducting electricity as the moisture in the wood. The whistling sound is the moisture escaping (as steam) as it's heated by the current until it reaches that critical point when a full blown arc appears across the lines.
It doesn't look like there is a tree tall enough nearby for a branch that size to fall across it with no leaves or smaller twigs.
@DerGreg: this here is another example- http://www.youtube.com/watch?v=kVXi_0H_ZzM
At such high voltages only ceramic blocks electricity.
looks cool but don't do it again
What interests me is that it doesn't arc until the wood combusts. Presumably the flame itself has very low resistance and started the arc.