The wires are all equipotentials, so electrons don't "build up" and must keep flowing. 

The total current going into a junction is the same as the total current going out of a junction.

The junction rule is only true in DC circuits.

The junction rule is only true where wires come together or split.

The junction rule is true for any number of wires. 

The junction rule is a consequence of conservation of charge in a circuit.