When the cockpit panel was recovered from the water, it was noted that the center tank fuel pump switches were off. I still fail to understand how a center tank pump can start an explosion when there is no electricity to the pump.
Good question, and the best theory (there's no solid proof) is that because of the decayed mylar insulation on the wiring, higher voltages from elsewhere in the plane's circuitry jumped to the pump wiring or (more likely) the fuel level sender unit wiring.
But the exact cause of ignition remains a mystery. There are several possibilities, but none of them is a solid lock. The official investigation admits this.
d.o.l.
Criminal Number 18F