From where? And what is 'high'?
High enough to arc through the insulation! Happens on spark plug wires all the time. Could come from lightening, St Elmo's fire, or static discharge. Doesn't necessarily need to be high current, just high voltage, maybe a few thousands of volts. In a moist environment, not as many thousands, as in a dry environment. There's been talk of the insulation being thin, which makes it even more likely to arc to ground.