The flagstand at the top of the hill is hard to see clearly from the track until you crest the top. The drivers were to wait until they saw the green flag in order to pass. Apparently either the 5th or 6th place car jumped before the green came out and attempted a pass in between the car in front of him and the pit wall. He scraped the wall, turned back in towards the track and tagged someone's rear end, spinning them. There are now 2 cars at rest in the middle of the track, which the remaining 40+ cars behind can't see. The red flag was now waving from a position halfway down the hill, but the drivers are all looking straight ahead at the track, not seeing it. 2 by 2, the rest of the field crests the hill only to find a pile of cars in front of them and no way to stop in time. Repeat x 10.
I didn't see it, this is the story I was told.
This pic is taken from the top of some bleachers at the bottom of 14 and isn't an accurate representation of what the flagstand looks like from the track surface. It will give you an idea of how steep the hill is, though. (Almost).