6. Why will too many electrical devices operating at one time often blow a fuse?

In an electrical overload, it leads to excessive heating of the wires and a possibility of fire.
In a short circuit, an accidental low-resistance connection between two nodes of an electrical circuit that are meant to be at different voltages. This results in an excessive electric current limited only by the Thevenin equivalent resistance of the rest of the network and potentially causes circuit damage, overheating, fire or explosion. Although usually the result of a fault, there are cases where short circuits are caused intentionally, for example, for the purpose of voltage-sensing crowbar circuit protectors.