Statistically, short circuits are the main cause for fires. A short circuit is described as a connection with zero resistance between two points in a circuit. This means that in that point there will be no resistance against the current passing through, that will be free to reach highest values. The obvious consequence to this is that the current, running through wires at hundreds of thousand amperes, reaches highest temperatures, high enough to melt the wires themselves. Hence the risk of fire or explosion.