SHC is a weird one, and, as far as I know, it remains totally unexplained*. There have been hundreds of cases reported, most with similarities.
Forensic investigations have attempted to analyze reported instances of SHC and have resulted in hypotheses regarding potential causes and mechanisms, including victim behavior and habits, alcohol consumption and proximity to potential sources of ignition, as well as the behavior of fires that consume melted fats. Natural explanations, as well as unverified natural phenomena, have been proposed to explain reports of SHC.
Usually, the victim’s torso has burned away entirely, with only a small area of charring to the surroundings. Often, limbs also remain, scorched at the end but intact otherwise. Just a couple of cases:
In 1986 retired firefighter George Mott’s remains were discovered in his New York apartment. According to some sources, all that was left of him was “a leg, a shrunken skull and pieces of his rib cage.”
Mary Hardy Reeser spontaneously combusted while sitting in a chair in her Florida home.
Upon discovery, all that remained was the burned seat springs, some bones, a shrunken skull and one foot that was still wearing its slipper. Again, the burning was localized.