What causes the degradation of the flight control laws to mechanical backup mode?

The degradation of the flight control laws to mechanical backup mode occurs primarily due to a complete loss of electronic flight control signals. In the A320, the flight control system relies on a sophisticated electronic architecture, which includes electronic signal processing to manage the aircraft's control surfaces. If there is a total loss of the electronic signals that govern these controls, the flight control system cannot function as designed, thus requiring a transition to a mechanical backup mode that allows for basic control of the aircraft using conventional means.

This mechanical backup mode is crucial for maintaining controllability when all electronic systems fail, as it utilizes physical linkages to the flight control surfaces, independent of the electrical systems. While the other options may lead to various failure scenarios, they do not directly trigger a shift to mechanical backup mode as definitively as the complete loss of electronic signals does.