Having just searched and read up of various auto and domestic/industrial sources, it seems that the usual cause of failure would be the bulb (burner) leaking the halide gasses and then the light producing arc can't be maintained across the electrodes. Then, the ballast trying to re-ignite it due to cheap kits having no way of sencing that the burner has failed. Or, just complete failure of the burner envelope causing very hot bits to contact the surrounding plastics. With burner temps in excess of 1000'C and up to 1300'C, and a circuit trying to continually re-establish the arc, its easy to see how a plactic headlight housing could then catch fire.
The other thing mentioned was a mismatch in the burner/ballast which can cause the burner to run much hotter than intended.
I'm glad I replaced my kit now as the light had started to flicker.
