A tube has components that can oscillate (physically) that movement will alter the signal going through the tube. A pickup is designed to do that
Microphonic implies the recovered waver resembles the original one. I can see how slapping a cable could possibly cause a resistive connection but this is still a stretch.
"Microphonic" in this context just means that physical movement or stress of a component, can cause audible perturbations in the components electrical signal path.
I've seen microphonic PCB components that made noise when you tapped them with a pencil eraser, and even solder joints there were probably more intermittent than microphonic.
If a wire like a mic cable is passing DC current (like from phantom power) and wiggling the wire alters it's DCR that could cause noise, I have also heard of, but not personally experienced issues with funny cable insulation properties (blamed on distributed capacitance or even piezo effects, but like I said I have never experienced this).
I don't get the physics
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While I never took them apart to confirm, I would speculate that microphonic components (like capacitors) I've encountered had loose bits inside there weren't supposed to move around, and when they did it changed the C's characteristics causing electrical noise in it's path.
I've encountered this with high gain preamps where I would tap around the PCB with a pencil eraser to find the rouge part (it would be the loudest). Since the circuit is powered up while you are doing this a non-conductive pencil eraser is the right tool for that job.
JR