In Chapter Two, when George asks why Whitey quit, Candy gives him a very general and very vague answer. In fact, he says that Whitey "just quit, the way a guy will." Candy then goes on to offer a number of possible reasons for this decision, including the quality of the food that is served to the men on the ranch and the general desire to move on to a new place.
It is interesting to note that Candy also changes the subject very quickly by reminding George that the boss was mad when he and Lennie did not arrive on the ranch that morning. This suggests that Candy is keen to distract George's attention from Whitey and the reasons which contributed to his decision to leave. This also creates some tension between George and Candy while suggesting that there is more to this ranch than first meets the eye.
Just because. Candy says that Whitey picked up his check and moved on - gave a couple token excuses, but just wanted to move on down the road.
These men are part of a pretty transient society always looking for the greener grass. It was pretty common for them to move from job to job for no reason other than to just keep moving. This is actually in stark contrast to Lennie and George who wish to stay in one place for a while to build up a stake but have to keep moving on because of the bad things that Lenny does.