In recent years, as you note, catalytic converters, power lines, welding equipment, cigarettes, mower blades, flat tires (rim scraping on asphalt), hammers, and broken glass have all started major fires. A Redding fire a few years ago was started by a man mowing his property -- the steel mower blade struck a rock and ignited a major blaze. Which is incredible enough, but the fact is that this risk was known, mowing at that time was prohibited, and he was prosecuted and convicted.
The fuel load is so increadibly dry, and "red flag" conditions, with high heat, low humidity, and gusty and often unpredictable winds, that any point of ignition can and will set off a blaze. Attempting to remove all points of ignition simply isn't sufficient or possible, though it may help in some cases.
A few cases and ignition causes mentioned here:
https://www.nytimes.com/2018/08/20/us/california-wildfires-h...