I was there.. about 100 feet back from the stage. It was surreal, and not something I will ever forget.
Cheap Trick was playing as the storm came in and winds picked up. I guess the organizers thought something was up so they cut power to the big speaker towers on the sides of the stage. The band kept playing the song, using only the amps they had in stage. They finished, waved to the crowd, and took off towards the back of the stage (the drummer dove under the risers). Before they got off the stage a big gust of wind ripped some of the scrim and roof off, and then the whole thing just collapsed backwards and down.
There were only three people hurt, and all of them are out of the hospital now. It was a real miracle. If it came the other way it would have fallen into the crowd.