The character John Burke sits in a chair all day because....?

From the text, we can infer that John possibly suffered a traumatic brain injury. The reason for his "mindless stupor", however, is not provided.

John Burke was what was known as queer-headed. Black and ageless, he sat rocking day in and day out in a mindless stupor, lulled by the monotonous squeak-squawk of the chair.