Recent research has been showing that native americans shaped their forests with fire. It's entirely possible that many of the north american forests intentional human caused fires as part of their normal cycle.

More here: https://en.wikipedia.org/wiki/Native_American_use_of_fire#Hu...

>Do we know that all human-caused fires would still have occurred "naturally" if the humans hadn't been there?

Does lightning stop happening if people are gone?

http://link.springer.com/article/10.1007%2FBF01998581

Humans cause some fires. Not all. I'm not even sure that humans cause the majority.

This varies greatly by region, land use, and weather patterns. In some places and years, humans dominate; others, lightning. Because humans tend to start more fires near cities, campgrounds, and other accessible areas, those fires tend to be contained or extinguished quickly. Lightning starts tend to be on ridgelines, near peaks, and in other remote areas that are difficult to reach. Many of the big, long-burning fires are lightning starts, but they also tend to threaten fewer structures. Obviously there are exceptions all around.

I think that's a combination of high winds and young growth. If it is young growth, it's actually illustrating what the article is describing, the young growth trees are young, but not at young s they should be, and they bridge the gap between the undergrowth and canopy. Once the canopy catches in a dense forest, there's a real problem, but a dense forest with undergrowth only on file may be a mild, slow moving (since the wind may be mostly blocked) fire.