my gut tells me that something smells awfully fishy here

Your analysis is mathematically first rate; the only problem is, these types of tandem fires have happened, like, a dozen times, in my own memory. All during these periods of hot dry winds in early fall. That seems to suggest natural causes rather than a regular epidemic of "fire bugs". After all, in south east they regularly get hit by multiple tornadoes per season and multiple hurricanes/flooding per season. Can't be "Water bugs" or "Wind bugs".