The fire will have been caused by a build up of heat - it may have been caused by overcurrent but a fuse would likely have blown before the fire began (that's why we have fuses).
It's more likely that a high current wire became lose in its terminal (or was not properly tightened) - to begin with, sparks would have jumped across a tiny gap causing blackening and sooting of the connection and a consequent high resistance joint.
Over time this bad joint would have become worse, allowing current to pass and the cooker to operate normally but slowly building up more and more resistance and becoming a small local heater in the connector block.
When sufficient heat built up things began to melt. It happens - bad enough in a domestic appliance but very dangerous in aeroplanes - see Air Crash Investigation Swiss Air 111.
Peccavi, July 2009
