The two episodes alluded to in the question both take place in a clearing where "[R]abbits come out of the brush to sit on the sand in the evening" because it is so peaceful. In Chapter One, George and Lennie camp out the night before going to work on the ranch, and George tells Lennie to gather some dead willow sticks so that he can cook beans for them to eat. Before he leaves, Lennie says, "I like beans with ketchup." George merely informs his partner that he has no ketchup. However, after having scolded Lennie about chasing a mouse and having to send him back to retrieve the willow he dropped in the process, George is exasperated with Lennie. So, when Lennie asks for ketchup again, George explodes, declaring how much easier his life could be without Lennie. Yet, when he sees Lennie's "anguished face," George is ashamed of his vituperation; he, then, recites for Lennie their "dream" of owning rabbits and having a farm, and the scene ends on a very peaceful note as the two friends sit together.
In contrast to this innocuous episode between Lennie and George who has become angered over an insignificant thing, but ameliorates their relationship, in Chapter Five Lennie returns to the clearing and fearfully hides in the bushes as George has instructed him. For, this time matters are much more desperate than catching a mouse or wanting ketchup. As he hides, Lennie imagines his Aunt Clara scolding him about his mistreatment of George;Aunt Clara disappears, but in her place is a gigantic rabbit, sitting on its haunches. The rabbit speaks to Lennie "scornfully," disparaging him and saying "[George] is sick of you." It tells Lennie that George is going to leave him all alone because Lennie is crazy: "He gonna leave ya all alone. He gonna leave ya, crazy bastard."
This is Lennie's greatest fear, to be abandoned by George. When George apologetically relates the tale of owning the rabbits in Chapter One, he allays all of Lennie's fears; even then, Lennie did not fear George's leaving him. But, in the final chapter, Lennie's fear is huge; he worries that George will be mad and will leave him and, ironically, it is a huge rabbit who tells him that George will abandon him.