As Katrina says in the Comment to this post, the fire was caused by a propane explosion in the staging area of acl. It was to the north of the AT&T stage, just behind the port-a-potties. Two people were critically injured, according to the news reports I have read. But, the public was never in any danger. ACL staff and police did a great job in controlling the crowd and containing the hazard so that this did not become a catastrophe. Firefighters were on the scene quickly, and the fire was put out in about 10 minutes.
I was at the Pete Yorn show when this all went down. He continued his set, until officials stopped the show briefly to get the crowd to move away from the area of the fire.