Wow. Last time I was up there (couple months ago), the main shooting area (ie the ditch with the hill behind it) wasn't exactly ready to burst into flames... just a lot of dirt, so you could shoot tracer all day long if you wanted to waste it on that. If it was really a shooter that started it, they must have been doing something that was just begging for trouble, and/or shooting somewhere other than the main area. I'd be more suspicious of the 4x4 guys... I've always seen much higher numbers of them up there, and they're the ones dragging the engines and exhausts of their homemade contraptions through the brush.
Who knows... it could have even been a careless smoker. That reminds me of the time that I was up a little North of that area on 36 and saw a lit cigarette butt fly out the window of a Hygene Volunteer Fire Department truck, towards dry brush on the side of the road. Needless to say, they lost any respect I had for them.