Hard to tell in the video but it looks like he might have loaded up the lee rudder and lost authority due to ventilation.
I agree - at about 40 seconds in on the wipeout video, you can see the windward rudder at a pretty high angle as the skipper tries to head down, but the boat doesn't respond. The rooster tail behind the lee rudder also tells the tale. Blowing the gennaker only worsens the pressure on the rudders, causing the boat to start heading up and over.
