How can moving my foot possibly affect electronics meters away?
The case of your microphone is probably electrically connected to the flexible membrane inside the microphone, and is responsible for grounding it, while the "ground pin" is only used to provide the return path for the FET preamplifier. In normal operation, both need to be grounded so that there's a complete circuit to drive the gate of the FET.

With the case "floating", the microphone is no longer senstive to sounds, but it does become very sensitive to external electric fields of any sort. Walking around, or even just lifting your foot, can create large amounts of static electricity, particularly in a dry environment, and the field associated with your body was affecting the case of the microphone. Your body might have a potential of several thousand volts, and it only takes a few mV to drive the gate of the FET.