It's an AC line, so when the helicopter retracts it will get a static charge instead of fluctuating in sync with the line, giving an immediate potential difference.
I have no source for this, just amateur reasoning.
It's very likely capacitance. At 200kV even a couple dozen meters above ground the capacitance would be enough to get a couple microamps over an arch (it would likely tickle a bit).
AC makes isolating rather complicated since increasing your resistance to ground naturally turns you into a capacitor which acts like a resistor in AC (which is why thin rubber soles on your shoes don't protect you, you want think ones to increase resistance and decrease capacitance) so a small current will flow.
At high voltage this small current can still be transmitted over an plasmaarc.
I have no source for this, just amateur reasoning.