Why do fires always start on the side of freeways?

This is the 80 just east of Pinole in Northern California. The photo doesn't do the scene any justice because I could feel the heat through the window as I drove past. This, despite my air conditioning.

There were no FD present yet and there were homes endangered just atop this small hill. I wonder. Is it always careless smokers flicking their butts out the window? I see patches of burnt often just off highways and fail to conceive of an alternative cause.