What did Lennie do in Weed that caused him and George to run away?

In John Steinbeck's novella Of Mice and Men the author tells us that George and Lennie had been working in the small northern California town of Weed. They are itinerant farm workers traveling around the state. In Weed, Lennie saw a girl in a red dress and reached out to touch it. George describes the episode to Slim in chapter three:

"So he reaches out to feel this red dress an’ the girl lets out...

In John Steinbeck's novella Of Mice and Men the author tells us that George and Lennie had been working in the small northern California town of Weed. They are itinerant farm workers traveling around the state. In Weed, Lennie saw a girl in a red dress and reached out to touch it. George describes the episode to Slim in chapter three:

"So he reaches out to feel this red dress an’ the girl lets out a squawk, and that gets Lennie all mixed up, and he holds on ‘cause that’s the only thing he can think to do. Well, this girl squawks and squawks. I was jus’ a little bit off, and I heard all the yellin’, so I comes running, an’ by that time Lennie’s so scared all he can think to do is jus’ hold on." 

Lennie is obsessively drawn to soft things which he likes to touch and pet. The girl accuses Lennie of raping her and the men of the town chase George and Lennie, who have to hide. George goes on with his explanation:

“Well, that girl rabbits in an’ tells the law she been raped. The guys in Weed start a party out to lynch Lennie. So we sit in a irrigation ditch under water all the rest of that day. Got on’y our heads sticking outa water, an’ up under the grass that sticks out from the side of the ditch. An’ that night we scrammed outa there.”

The scene in Weed also serves as foreshadowing for chapter five when Lennie is alone with Curley's wife. While Lennie is stroking the girl's hair she tries to break away and, as in Weed, he holds on. Eventually her struggles scare Lennie and he accidentally breaks her neck. His obsession with soft things and his powerful strength finally contribute to his downfall.