Why did George and Lennie have to flee from Weed?

The text is rather circumspect at first about the reason George and Lennie have to leave Weed. We know that they worked together on a ranch there, and George warns Lennie that when they move on, he must not "do bad things" like he did in Weed. Lennie seems not to remember exactly what these "bad things" were, but he does remember, "triumphantly," that he and George were run out of town. They did not have any choice about leaving: the townspeople forced them to go. The people of Weed were "looking for" George and Lennie, presumably with the intention of punishing them, but didn't succeed in catching them, as George encouraged Lennie to flee instead.
Later, when asked why they left Weed, George simply says that the job they had been working on "was done." Eventually, he admits to Slim that Lennie had assaulted a girl in a red dress in Weed, not knowing really what he was doing, but wanting to touch her. The more scared the girl became, the more scared Lennie became, with the result that he clung on more and more tightly to the girl's dress. The girl then told the police that she had been raped, at which point the men of the town set out a party to lynch Lennie. Lennie ultimately "didn't hurt the girl none," but she was sufficiently afraid of him to call the police. Lennie, because of his sheer size, cuts a frightening presence; although, ultimately he is harmless.