AI4QJ wrote:
I think in this
case the insulating material broke down due to high voltage.

J. C. Slater agrees with you. In "Microwave Transmission",
he wrote: "Particularly in coaxial lines, the limit to the
possible transmitted power is generally set by difficulties
of corona or sparking between the two conductors of the line.
The greatest practical reason for avoiding reflections is to
reduce the magnitude of these voltage maxima."
--
73, Cecil http://www.w5dxp.com