Most theorists agree that it is a karmic occurrence, brought about by the disruption of peace in the universe, by a growth in the crust punk trend, specifically in the American Midwest.
Guitars tend to explode at a high enough temperature to ignite dirty clothing and greasy hair, so it is typical for the person holding to guitar to catch fire as well. Not surprisingly, there has never been an incident recorded where someone has tried to extinguish a victim of SGC, though many have admitted to thanking God after witnessing the miracle.
Researchers have been trying to establish a cause-and-effect relationship between SGC and the Mayan prediction of the 2012 apocalypse. Nostradamus' prediction has already been connected when astronomers discovered a constellation depicting SCG, perfectly situated with the predicted alignment of the planets on the day of the Rapture.
The majority of recorded cases have occurred in coffee shops, to victims who have been described as crust punk, gutter punk, and hippy.