At various times throughout the book, George refers to Lennie getting in trouble on a regular basis, and he tries to condition him not to, or that, if he does, he remembers to come to their spot at the river where George can find him. He also talks about it to Slim later in the book, telling him "what happened up near Weed".
Lennie's past is that he doesn't know his own strength, and in his ignorance, and in his curiosity, he scared a woman in Weed. She started screaming and he wouldn't let go. This set them both on the run from dogs and posses.
It is this same curiosity and this same ignorance which gets Lennie into trouble again, accidentally killing Curley's wife at the end of the story. We understand why it happened when we read it, we sympathize with Lennie, and George and almost everyone else because we know he's not a killer, but like Slim, we know there's nothing to do but go get him.