Heavy rains in that area last year led to a lot of new growth. It becomes quite flammable after winter kills it all. Have someone throw out a cigarette butt on the highway when there are 50 mph wind gusts and viola.
Yeh, the source could have been any number of things, ie; cigarette butt, welding sparks, safety chain dragging on pavement, catalytic muffler, lightning, power lines being whipped around, somebody grilling, etc, etc. could go on forever…