Well, there are two or three things that could have happened, the most obvious would be a spark from the open fire, hitting him and igniting his clothing, although unlikely it is totally possible. Also he could have just fallen asleep with a lit cigarette, it burned down and lit his clothes, and he wakes in flames, jumps out of the chair and falls to the floor, with the cigarette burning itself out.
The problem with these two possibilities is that they leave no evidence of what caused the burning, hence the confusion.
The problem with these two possibilities is that they leave no evidence of what caused the burning, hence the confusion.