How does the account of Whitney's quitting contribute to the book's mood of alienation?

The short answer is "just because". Candy explains that Whitey used to use George's bunk and was paranoid about bugs. One day Whitey picked up his check and moved on. He gave some of the usual excuses but in the end he "just wanted to move on down the road." Whitey is an example of the transient life that these men live. This represents the very isolated and lonely life of these itinerant workers. Few bonds or friendships are made. Men stay for awhile and move on and repeat again at a different ranch or work farm.