I could hazard a geuss and say it could have been caused by an aftermarket HID kit. Ive heard of it happening twice to folks I know, one was an aftermarket fog light on a blob eye skooby. Again on a Golf GTI headlight. The skooby was minor and just burnt the bumper but the GTI went up like this, the increased power draw had deteriorated the wiring behind the light causing it to immolate when power was passed through it.
The wire on the GTI was running rather close to something flammable (I think it was the fuel filter although im not sure) and it went up like this mini.