What factors caused Muley to be "touched"?

From the text:

Muley said over the fire, "You fellas'd think I'm touched, the way I live."
"Touched nothin'," said Joad. "If you're touched, I wisht ever'body was touched."
Muley continued, "Well, sir, it's a funny thing. Somepin went an' happened to me when they tol' me I had to get off the place. Fust I was gonna go in an' kill a whole flock a people. Then all my folks all went away out west. An' I got wanderin' aroun'. Jus' walkin' aroun'. Never went far. Slep' where I was. I was gonna sleep here tonight. That's why I come. I'd tell myself, 'I'm lookin' after things so when all the folks come back it'll be all right.' But I knowed that wan't true. There ain't nothin' to look after. The folks ain't never comin' back. I'm jus' wanderin' aroun' like a damn ol' graveyard ghos'."