So volume 24 that just released overseas includes a section that sheds some light on SHC.
"The Empire of Tokyo branded this supernatural phenomena 'Spontaneous Human Combustion'. In truth, the workings of this kind of occurrence are far more complex. What appears to be infernalization (the process through which a human being morphs into a fire demon) is actually a forced fusion, of the real being and a mirror image of that specific individual, a 'doppelganger'. Doppelgangers are the result of humanity's perception. The reason why doubles in general sport a demonic appearance shrouded in flames is because humanity perceives themselves as wretched and twisted monsters. As such, when a doppelganger crosses over from the world where human thoughts materialize, it searches its identical real self through heat, like a moth drawn to a flame. Upon finding its real counterpart, the double forces a kill and replace and absorbs the human's consciousness. The forced fusion causes the doppelganger to gain humanity, and as a result they quickly become self-aware of this twisted form, and instinctively treat any outsider as a potential threat.
Those who have developed ignition abilities also gain awareness when their doubles are close by, allowing them to defend themselves and either overwhelm the shadow or perish. Common people do not have that advantage and risk becoming infernals and there's nothing they can do about it.
But either way, the rule is that when two identical individuals meet face to face, it will result in the death of one of them."
I summarized it as best as I can because I don't have access to the volumevolume rn.