It seems odd that he fell without his backpack. Perhaps he stop to get layerdown, or something; took his pack off and didn't had his iceaxe within reach, slipped and that's all it took. Right there at the 400s, if you don't get to selfarrest within a few seconds, it's over. You'll pick up so much speed that if you don't selfarrest right away, there's nothing you can do
