It was started a a careless mountain man and his damn flintlock.
Theory 1: He put too much powder in the charge, and the excess ignited weeds.
Theory 2: the wadding or bullet patch was still smoldering when it hit the ground.
Theory 3: he missed the bear, was running for his life, the flint fell out of the lock, and hit a piece of high grade iron ore, struck a spark that just happened to land in dry tinder.
Theory 4: He was a really a tenderfoot mountain man wannabee who forgot to keep his powder dry, so started a small fire to dry it out....
And thanks for both the ping & the link to the other mystery; I had never heard about the "missing eclipse" before.