how did this accident happen?

[Muzz]

1) He apparently was riding outside of his comfort zone. I think they were pushing it, and sometimes the guy right behind you can be stressful in making the lead bike think "I'm boring him and need to speed up". This is one reason I only ride with a handful of people in the twisties.
Luap

The guy that canned actually roared past at the start of the video and took the lead. Probably felt he had to justify doing that.

Those still shots were enlightening.

[PJPR01]

Looks like he's doing about 60 mph on a 20 mph curve (estimate from his speedometer and compared to the sign right before the curve)....he took one look at the turn and stood it up...just a plain panic situation.  Seems like 2 x the posted speed limit is what most folks are comfortable with... at 3x...that was beyond the skill level.

No idea how heavy his backpack is and I know some folks do ride with one, but if there's a lot of weight in there, that can be a barrier to moving quickly around on the bike as well...momentum being the factor there, no idea if that contributed here to him pulling up on the bars and straightening out.

[keener]

he went where he looked ........., he wasnt to fast , he didnt have a bad line , he looked at the guard rail and rode to it and crashed ...simple ..
you go where you look ....

watch the vid  and look through the corner...if he would have done that he would have been fine

[keener]

Also target fixated...

When he panicked, he was staring at the guardrail, where he should had been looking down the curve

 :1:      you go where you look

[sidecarnutz]

I'd call it multi layered foolishness as the cause.

[redrider90]

He choose a BAD line plain and simple. Add to that  like some earlier said he was taking a 20 MPH curve (decreasing radius) at 60 MPH.
Where would you guys begin your line in this curve and would dive into full tilt this early in the curve?

[sidecarnutz]

Random crap like the dark van pulling over to let them fly by ruined their day.  Had it stayed it's course slowing the group down they probably would've made it home ok.

It was the van's fault.

LOL!

[Darren Williams]

Looks to me like a typical sport riding off. The guy was probably a decent rider that just got a few things wrong in a corner and then it all went wrong (he bailed on it). Happens to people who are going to ride in a sporting manner. The better riders, who don't crash in that instance, are the ones that recognize the first error early and don't compound it by making more. As we get older, and our reflexes and mental processing slow down, those kind of things are likely to happen more often than when we were younger and on top of our game. That is as long as we get by the learning stage when young and indestructible.

Personally, I don't do much more than double the corner speed signs and that's only on a road I know. If I don't know the road, it's usually a bit less. And my trail riding has slowed to a crawl anymore. Getting older sucks!

[M0T0Geezer]

Searching YouTube for "motorcycle crashes" is an unending source of both education and head scratching for me. 

The proximate cause of almost every crash is a rider going [way] too fast for conditions [traffic, weather, visibility, traction, etc etc). 

I'm undecided if secondary involvment in someone else's crash, or a random attack from some cage, always qualifies as one's own fault.

'Geezer