Got there immediately after they closed the road. Coming from Stevens Creek/Redwood Gulch, it was a maybe a mile west. Spoke to the rider's buddy briefly. Don't know cause of crash. At the time (roughly 10/10:15am), a fireman told us "it's bad," but that the rider was responsive and that the road would be closed at least 45 minutes in both directions.
We were trying to get to Big Basin and decided to backtrack 9 East > 17 > Bear Creek Road into Boulder Creek. Incredibly, we wound up witnessing another crash, this one much more minor on 236 in Big Basin, about a mile from the park headquarters. An unlucky solo rider on a Bonneville came around a blind corner and was met with a lot of washed out gravel and sand. He wound up hurting his shoulder (possible fracture), but was otherwise ok. My buddy and I stayed with him and helped direct traffic for the first responders. Even more incredibly, the Highway Patrol LEO who showed up had been one of MANY who worked the first, big crash on 9. He told me that that rider suffered two broken legs. The LEO also said it's been a particularly bad week for crashes in that area (cars + trucks, included).
Please note that all of my info is hearsay from first responders! Wishing the rider a speedy recovery.