The video is over 15 minutes long but the incident starts at the 09:55 mark. You might want to skip to that point.
At around the 10:00 mark, after the car has slowed down, it passes through the accident site.
If you look at the video closely (close to the top right corner of the car's rear windscreen), at the 10:32 point, you can see the second crash taking place which supposedly involved the BMW M3 "Ring taxi" hitting the back of the Corvette.
Although we didn't get to see the first accident taking place but at least it gives us a clearer picture of what happened during the second one. A long strecth of a fast left bend was just before the accident site and as we can see from the video, many cars scrambled to apply their brakes after seeing other vehicles slowing down. And any cars that were too fast would have little time to brake and avoid a collision from happening.
The best question for me to ask is "Why was there road works done on a track day?"
link to the accident
Photo credit: jalopnik