The coupler changes the apparent length of the antenna. If the impedance is not matched correctly a standing wave is formed (superimposed on original carrier but travelling in opposite direction) which reflects power back to the transmitter, which can burn out the last amplifier stage. Incidentally when you hear yourself talk through the headphones this sidetone is tapped off at this point - hence no side tone = burnt out transmitter. The reason they use 1/4 wavelength is that this is the point of maximum power in a sine wave ie 90deg. Most aircraft these days have 2 systems feeding into a notch antenna. When you transmit on HF1 for example, a relay opens in HF2's coupler to prevent the power from HF1's transmitter going straight into HF2's reciever. These relays fail from time to time as well and one HF will lock out the other.
