This answer is interpretive and contains a speculative analysis about Lennie and his actions.
There are many references to an incident in the town of Weed. George does not really know what happened because he was some distance away when the girl started screaming. Then he and Lennie had to run for their lives, so the only report he got was from Lennie. Lennie claims he only wanted to feel the fabric of the girl's dress, but later evidence suggests perhaps Lennie is developing an interest in sex and that his strong interest in petting soft little animals has been a budding interest in sex (which Lennie doesn't understand). When he begins petting Curley's wife's hair in the barn, perhaps he becomes sexually aroused, increasing his confusion and the violence of his reaction. It is very significant that George says the following words when he sees the dead girl lying in the hay in the barn:
"I should of knew," George said hopelessly. "I guess maybe way back in my head I did."
The reader, too, should know that Lennie is going to continue to be unable to control his reactions and desires. George can't be with him all the time. George wasn't with him when he frightened the girl in Weed, and George wasn't with him when he killed Curley's wife in the barn. (George doesn't really know what happened in the barn. It looks very much like an accidental killing in connection with an attempted rape--and that might not be far from the truth.) Lennie doesn't understand and can't control his sex drive, and his enormous physical strength makes him especially dangerous.