If I'm not mistaken, they are foam blocks with reflectors mounted to them. I believe that the timing lights are located at the bottom of the concrete wall, and the reflectors in the foam blocks for the left and right lanes. That's why they can put new ones up quickly when they get hit (which they frequently do).
From the ESPN replays, it looks like Bernstein hit the foam blocks, one of which flew under Force's car, and the right rear tire blew out. Right around that same instant, the parachute came out, and some combination of forces (no pun intended) ripped the car in two. But I will leave the analysis to the NHRA - They do a darn good job.
Like most NHRA followers, I am a huge Force fan, and have been thinking quite a bit about him, the team, and his family. If you've never had an opportunity to meet this guy, you would be surprised by how friendly and genuine he really is. It is amazing how much time he will spend signing autographs and talking to fans. He is one of the most down-to-earth people I have ever met (in a sport that is full of great people), and one of the last true 'characters' in motorsports. I hope for a complete recovery for him.
-Tom