As the novel opens, Lennie and George are walking through the woods near the river. We learn that they have come from the town of Weed, where Lennie was involved in an incident that required them to leave hastily. This, in fact, is why they are running late to their next job--George chose to have the two of them get off the bus somewhere out of the way so they could not be followed.
This is the first of several events revealed in the first chapter that foreshadow later trouble. Lennie's inability to control his impulses ultimately leads to terrible tragedy for not only himself, but for George and others, as well.
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.