Why Microphonics is unwanted modulation of a circuit by mechanical vibration or shock. In an oscillator, physical movement of a capacitor's plates, a crystal, a coil, or even a circuit board flexing slightly changes the tank's L or C, which shifts the resonant frequency. The result is a warble or 'ping' on the signal when the rig is tapped or when nearby sound or vibration reaches it. Cures include shock-mounting the oscillator, using rigid components, and potting or bracing the tuned circuit.
Watch out The choices mentioning a microphone are traps based on the word root only; microphonics has nothing to do with microphones or microphone cables, though the effect is named after it because the circuit behaves like one, turning vibration into an electrical signal.
Microphonic = the circuit acts like a microphone: tap the rig and you hear the frequency wobble.