Why did George and Lennie have to flee from weed ?

Lenny likes soft things, and he inappropriately tried to touch a girl's dress. When she put up a fuss because she had no idea what he was doing, he panicked and held it tighter. All the men in Weed were after him, and they had to escape. That is why they left.

“Jus’ wanted to feel that girl’s dress—jus’ wanted to pet it like it was a mouse—Well, how the hell did she know you jus’ wanted to feel her dress? She jerks back and you hold on like it was a mouse. She yells and we got to hide in a irrigation ditch all day with guys lookin’ for us, and we got to sneak out in the dark and get outa the country."