This answer is in part a frame challenge.
"Spontaneous Human Combustion" has been scientifically shown to be no such thing.
It is a characteristic of fatty meats that if they are wrapped in cloth, which is then set alight in an enclosed area with a low rate of oxygen ingress that they smoulder, the fat melting and burning on the surface of the cloth like wax rising up a candle wick. In fact, this can lead to an otherwise non-flammable substance being able to burn.
This can be demonstrated fairly readily with an uncooked leg of pork and some woolen cloth. Wrap the pork in the wool, then light it in an enclosed space. The fat melts, and wicks through the wool where it burns.
Historical cases of SHC have in fact been cases where the victim has died at around the time that the fire starts - often from some source of ignition that a healthy, living person would reflexively avoid, like a cigarette, a heater or a cooker. However, being very ill or recently deceased, the "victim" is unable to move away or extinguish the fire, and if still alive when set alight, they die from the trauma shortly thereafter.
There have been recorded cases exactly like the OP's, where a person enters a room to see a flame suddenly erupt from the victim. This has been attributed to the fact that the victim merely smoulders when in a closed, oxygen-deprived room, but when the witness opens the door and enters, there is a sudden rush of oxygenated air into the room, which causes the smouldering fire to flare up in just that manner.
So... in such a case, a coroner might conclude that the victim met their fate as a result of some non-fire-related reason, or perhaps was incapacitated by some other cause, which enabled their clothed body to be set alight, causing their death.
The technical term applied to the case would therefore not be a medical term at all, but a fireman's description: a smouldering fire enabled by the wick effect, occurring in an enclosed, unventilated space, which upon the door being opened and admitting fresh, oxygenated air, caused the slow-burning oxygen-starved fire to flare up and burn more rapidly.
Any movement that might be attributed to the death of a living person at that specific moment where the smouldering flame flares up might be more accurately attributed to the suddenly increased heat of combustion causing post-mortem tissue contraction, movements and even vocalizations.
Any medical terminology used would be to describe (if there were sufficient remains to successfully autopsy) the cause of death or the cause of sufficient incapacity so that the victim could be unable to react to being burned sufficiently to avoid their death.
This could be any of a great many causes, from heart attack to a drug overdose, and is beyond the scope of this question.
The OP need not invent any implausible internal source of combustion unless future advances in technology enable a person to be poisoned in such a way that enables true spontaneous combustion from within.