You raise an interesting question. This is a dual chamber of horrors that conspire to give rise to this dreadful phenomenon which is thankfully rare. The largest driving force is almost always the deep subconscious mind agonizing over inner tormenting done by interlopers and fueled as well by awareness of karmic dilemmas in one’s own past, seen directly in the akashic records, and this is the energy and fear that is focused on an area of the body too intently and giving rise to generation of energy, in the form of heat, to reach a flashpoint and initiate combustion. This is always aided and abetted by cellular memory because it is the cellular memory that will focus it to a point, and this is what enables the energy to gain sufficient force and intensity to give rise to such high levels of heat needed for combustion to happen. The focusing by cellular memory will be in response to the instructions and intention of the deep subconscious, but play quite a role in providing the focus without which the intensity would be too diffuse to reach such high levels needed to set the body literally on fire. This also is another illustration of the power of cellular memory to take a negative impulse and worsen it. This is done on many levels to give rise to bodily symptoms. It is often that focus that gathers enough negativity in one place to generate a symptom that may well become chronic and worsen in intensity to become quite a nuisance and even intolerable.