What caused this burn out?

[ppppenguin]

I think I've related the story of the exploding cast iron joint box. This was at the top of a pole outside my aunt's house. It had been dripping pitch for a while and she'd reported it. One evening it went off with a very loud bang, red hot cast iron fragments everywhere. Blew the fuses at the sub. The leccy people worked overnight to restore power but my aunt was definitely not happy.