The guy in the white helmet directly in front of him took almost the same line through the curve and made it just fine; I didn't see his brake light come on. There does look like some dust and other loose sediment spilled over onto the road but it doesn't look THAT bad.

I think the crasher saw the decreasing radius... and maybe the oncoming rider... and hit the brakes at just the wrong spot. But again the road doesn't look terrible. Maybe worn tires was a factor; I can't tell if his rears have any tread on them at all.