It starts off with an ecosystem that has regular fires. Because it's arid, dead organic matter won't rot, so it just sits around until it catches fire from lightning or whatever, and then it burns up, clearing the land, releasing nutrients, stimulating heat-activated seeds etc, and all the fluffy little armadillos are happy and well fed as a result. Then some people come along, build a town or 10, get terribly concerned about the fires and start creating fire-breaks and building fleets of fire-fighting aircraft and the like, which prevents the regular cycle of burning and renewal from taking place, so the organic matter just keeps building up until eventually there's so much of it that when it catches fire, it completely overwhelms the fire defences and burns up the towns. Afterwards, people blame it on arsonists and rebuild their wood-frame houses, so it happens again and again. They just completely fail to deal with the reality that they've been complete retards in building highly flammable buildings in the middle of a fire-driven ecosystem.
That's my take on it, at least.