It was all those wooden houses that burned up.
Which is why they used incendiaries.
The Army was testing out bats (the animal - not divert their attention with baseball) with timed incendiary devices. Drop 100 bats in a “bomb” that opens up at 100-feet. When day comes the bats find a house or eaves to sleep in. Then the bat-bomb goes off and lights the house on fire. With 1000 bats causing spreading fires.
During testing, things went awry and some bats got loose, went inside a hanger, and burned it down. They shortly thereafter abandoned the idea. Seems to me that was sort-of proof that it would work!