I was watching the race Sunday night when they showed the wreck. One of his rear tires blew as he went through the traps at 322 mph. The frame broke between him and the engine, and then his car glanced off the left wall, glanced off the other car (didn't even damage the other car), and came to rest back along the left wall. There was a fire, but it was extinguished almost as soon as it flashed by the safety workers (no F1-style 90-second wait at the dragstrip). The report I saw said a wing strut caused the fatal injury.
On the whole, it didn't appear to be that catastrophic of a wreck. No sudden stop, no cartwheeling, etc. I've seen people walk away from worse. Just a freak occurence to receive that kind of head injury.