The plant was decomposing inside a wooden pot with a metal band. The decomposition in the pot caused heat, the outside temperatures exacerbated the situation, and at some point the wooden base of the flower pot failed which allowed oxygen in. These factors combined resulted in spontaneous combustion.
To keep this from happening at your home, if a potted plant dies, be sure to either continue watering the soil so it does not dry out, or till and turn the soil so the heat doesn't get trapped.
Here's to fireproofing the garden!
