How does Lennie end up in a fight with curley?

No they don't. Lennie crushes Curly's hand, but Slim persuaded Curly into saying that he got his hand caught into a machine,and in return nobody would find out how Curley, the winner of a boxing tournament called the 'Golden gloves' tournament, lost to a simple ranch worker. Curley's father only knows that Curley got his hand caught in a machine.