There is one explanation for the fire that they missed on that site which also offers an explanation for the two other huge fires that all started the same night in other parts of the midwest. I came across this explanation on Wikipedia.
"An alternative theory, first suggested in 1882 by Ignatius L. Donnelly in Ragnarok: The Age of Fire and Gravel, is that the Great Chicago Fire was caused by a meteor shower. At a 2004 conference of the Aerospace Corporation and the American Institute of Aeronautics and Astronautics, engineer and physicist Robert Wood suggested that the fire began when Biela's Comet broke up over the Midwest. That four large fires took place, all on the same day, all on the shores of Lake Michigan (see Related Events), suggests a common root cause. Eyewitnesses reported sighting spontaneous ignitions, lack of smoke, "balls of fire" falling from the sky, and blue flames. According to Wood, these accounts suggest that the fires were caused by the methane that is commonly found in comets."