555 timer circuit error
The missing resistor (known as a pull-up resistor) was the issue. Without it, there is nothing to define the voltage at the trigger pin, except when the button is pressed. This means that airborne electrical noise can couple in to the pin, which might go low enough to trigger the circuit. Since the circuit then latches on for a long time, it doesn't matter that the random noise might then go to a higher voltage again - one very short low trigger is enough to produce the behaviour you saw.

Long wires will definitely have increased the circuit's ability to pick up this random noise by acting as a better aerial (although not "better" for your circuit).