Can you explain why this happens when it goes above 120V? Is this simply a LV line to HV line coupling capacitcance? Or is a combination of the HV capacitance to ground and the LV capacitance to ground?
I can try, but I'm mostly self-taught and field-learned, not schooled like the engineers here. My understanding of electrical theory comes from reading, experience, discussions with my EE friend, and this place.
It's a bit of everything. When it comes to indirect coupling, meaning high-impedance capacitance or induction, the influence of everything comes into play, proportionate to the relative amount of coupling.
A wire physically placed halfway between a high-voltage line and earth would have half of the line-to-earth voltage
if the amount of coupling is equal, but the line is a wire, and the earth is, as you know, huge.
Hopefully, the brain trust will step in and add factoids that are more accurate than what I said above.