What is a possible cause of high voltages that produce RF burns?

For an RF ground to be effective and safe, it must present a low impedance path to ground at the frequency of operation. If the ground wire happens to be an odd multiple of a quarter-wavelength at your operating frequency, it behaves like an impedance transformer, presenting a very high impedance at the equipment end. Because the RF energy cannot easily flow to ground through this high impedance, high RF voltages build up on the radio's metal chassis. If you touch the case while transmitting, you become a better path to ground, resulting in a painful RF burn.