I had a look at Wikipedia, and it stated there were about 200 cited cases over 300 years. That's less than 1 per year. In most cases, there was a lack of thorough investigation and / or reliable witnesses. In modern cases, where photographic or other evidence exists, a cause can always be found. So I wouldn't take much notice of "spontaneous combustion" stories unless they can be verified.
That said, I think God does kill people from time to time. He did it in the case of Ananias and Sapphira, and while I can't prove it, I believe that if He thinks someone is going to cause too much trouble, or has already done immense damage, I believe He'll occasionally act. But since he let people like Adolf Hitler, Josef Stalin and Pol Pot live out their lives with all the damage they did, then I don't think He does it very often.
I suppose one case that comes to mind goes back a few years. The story is that an alcoholic who brutally abused his parrtner's two children, murdered them and then forced her to help him burn the bodies and bury the remains.
Not long after he was killed by a truck colliding with his vehicle, and never went to trial. I sometimes wonder if God helped him to plant his foot on the accelerator when the alcoholic murderer was waiting to pull out, when normally the bloke would have braked.
That's my suspicion in his case. But I don't place much credibility on "spontaneous combustion" of humans.
However this does remind me of a story my sister told me. Not long after she and her husband had built their new house in a new acreage estate, they went to a Christmas party. But when they got there, she had this persistent nagging feeling that she may not have set the alarm. She told her husband, and said she was going back to check.
He told her to stay with the kids, and he'd go back. As he got close to their house, he could see a light underneath (it was high set). At first he thought there might be an intruder with a torch, but as he got closer he could see flames licking up the side of the laundry. He put them out, but the laundry walls were scorched.
Apparently he'd been oiling the floorboards, and thrown the rags in a cardboard box next to the laundry. Unknown to him, in fine print, the oil stated that impregnated rags could self ignite if left together, which in this case they did.
My sister's comment was "Somebody was looking after us that night!" (and she wasn't formally religious). First there was the persistent nagging sense about the alarm. Then there was the timing. Had her husband been five minutes earlier, the flames would probably not been evident. He'd have set the alarm and returned to the party.
Had he been five minutes later, the flames would probably have had a very good grip on the house, as it was basically wood, and being in a new estate, there were not many other residents to raise the alarm.
I think my sister wondered Who kept nagging her about the alarm.
So if you're intending to oil any floor boards, read the fine print.