It is entirely possible for a piece of equipment, or even an outlet or light switch, to develop a very minor short in it. Just a trickle of power and nowhere enough to trip a breaker.
But it can be enough to heat the device or wires and, very gradually, deteriorate the insulation. Over time it gets worse and worse until there is enough heat being produced to start a fire.
Animals, perhaps mice or termites, even a bees nest, can cause damage to wires with the same result - a minor short that is enough to build heat over long periods to the point that a fire starts. I once worked an industrial site that had a large well with automatic switch gear in the well house to start and stop the big pump. A squirrel got inside and got across the main, 480V, terminals. It was electrocuted of course, but remained there, causing a short between the two terminals. The resulting fire was contained within the metal box, but the entire relay had to be replaced.