There looks to be a bit of green corrosion on that wire, am I correct? That much doesn't build up so fast. Could have been a cut in the insulation and as the corrosion degraded the metal it got to the point the wire was too thin & finaly let go under load.
One possibility anyway. Usualy, other than a section of fusable link, if a wire gets hot enough to burn out more of the insulation is burned up than that one section shown there.

1956 GMC 370 dump " 'Tater "
1970 VW Volksrod "the Black Bomber"
2007 Chevy Avalanche
2020 Jeep Cherokee Trailhawk,
2005 Harley Davidson FLHTCUI

I dig all cars, old and new, whether they were hammered out of American iron, German steel, or Japanese tin cans. Being unable to appreciate them all is missing out on a world of great things.
But thats just MY opinion! :P