Most people who die this way are depressed, elderly, alcoholic smokers. This is an obvious explanation for how they combust.
But of course the burned remains are strange. Feet and legs are left with flesh on them, while even the bones of the rest of the body are turned to ash, which normally takes a long time in a cremation oven. Another anomaly is that the home of the victim is generally left untouched by flame.
I think this is caused by the wick effect. Intoxicated and unhealthy, the victim ignites themselves while smoking. Unable to do anything about it they burn quickly. Their body fat and clothes make great fuel allowing the fire to burn to a temperature high enough to turn bone to ash. The fire goes out when the fat and clothing is burned away and the house is more or less untouched. The reason certain limbs of the body may remain is because they were unclothed.
So there, it's nothing really out of the ordinary, just very rare and terrifying.