It should be noted that George never really saw what happened between Lennie and the girl in Weed, just as he never really saw what happened between Lennie and Curley's wife in the barn. When George is telling Slim about the incident in Weed he says:
"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."
After that they are both on the run, so George has no occasion to get any account of the incident other than Lennie's. When they finally had a chance to stop running and talk about what happened, Lennie must have told George that he only wanted to feel the fabric of a pretty dress. This is bad enough in itself. A strange man can't just go up and start handling a girll's dress, but Lennie is naive and doesn't grasp this fact, or the implications of committing such an act. It is hard to imagine exactly how Lennie might have done it. Would he try rubbing the fabric or gripping some of it between his fingers? The plain fact--as George realizes only too late--is that Lennie was more attracted to the girl than to her dress. When George is looking down at Curley's wife's dead body in the barn, he says:
"I should of knew. . . . I guess maybe way back in my head I did."
What he should have known was that Lennie must have actually gone up and grabbed the girl. It was a sexual assault--although Lennie may not have realized exactly what he was doing, because he did not understand his own feelings. That would explain why this girl reported that a strange man had tried to rape her and also explain why
"The guys in Weed start a party out to lynch Lennie."
Weed is a small town in the far northwest corner of California. It is several hundred miles north of San Francisco, which is at least a hundred miles north of Salinas. So the fact that George and Lennie have traveled such a long distance before looking for work suggests that the incident in Weed was extremely serious. They could have both been killed by the lynch mob or alternatively thrown in jail for assault or attempted rape.
While they were working in Weed, Lennie saw a girl wearing a pretty red dress. Focused on the dress's beauty, he reached out to touch it, and the girl screamed. Lennie panicked, and holding on even tighter, would not let go. George had to "(sock) him over the head with a fence picket to make him let go" before he would release the dress. The girls eventually reported the incident as an attempted rape, and "the guys in Weed start(ed) a party out to lynch Lennie". George took Lennie to hide in an irrigation ditch, and when darkness fell, they fled the vicinity.
Lennie did not actually hurt the girl, but, being big and strong, he scared her badly. As Slim observed, "(Lennie) ain't mean", and George agreed, explaining, "(Lennie) jus' wanted to touch that red dress like he wants to pet them pups all the time".