To answer this question, take a look at the first chapter. According to George, it was the fault of the bus driver. When he dropped George and Lennie off, he told them that the ranch was just a "little stretch down the highway." But he lied: the ranch was much further away, forcing George and Lennie to walk for miles on a very hot day.
Later, when George and Lennie arrive at the ranch, George tells this same story to the boss. He says that the bus driver gave them a "bum steer," for example, which forced them to walk ten miles.
Looking deeper, this problem with the bus is a sign that life on this ranch will not be as easy or as pleasant as George and Lennie first hope. It, therefore, foreshadows the difficulties that the two men will face, like their confrontations with Curley, and Lennie's tragic relationship with Curley's wife.