What caused this burn out?

[ppppenguin]

Looks like an 11kV 3 phase circuit. The 3 red cables each have a single core and the pole is a transition from underground to overhead. All very commonplace. The red cables would have replaced older ones, probably paper insulated lead sheathed. My guess would be that one of the old cables developed a fault and overheated. This wouldn't necessarily have triggered a protection relay in time to avoid the damage seen.