I don't know if this has been discussed before.
It says that when he died, his fiery spirit burned in him, but mabe Tolkien was thinking about the idea of
Sponatneous Human Combstion. As we know, though he was a devout Catholic, he liked hearing stories of strange events and the occult.
Maybe he was just playing with the idea, as the phenomenon had never actually been proven, and maybe he was adding his own readon and a case for it.
Just an interesting thought.