Ouch! All that I can think is that he was on the throttle a bit at that point, and there seems to be a noticeable dip in the road just before he looses the rear, almost as if this was just enough to unload the rears with the added power. The painted arrows are only after when he is already sideways, so not that. It does seem like he was "sleeping" too in terms of his reaction, or lack there of...
Very sad to see this happen to a car like this!