Curleys wife is being a burden. She wasn't loyal, and she's trying to get at the workers. That is also the reason she ends up dead.
Curley's wife enters the room, and they talk about how they enjoy touching soft things. She tells him he can touch her hair, but when Lennie strokes it too hard and messes it up, so she gets angry. She tries to jerk her head away, and, in fear, Lennie hangs on to her hair. Curley's wife begins to scream. To keep her from screaming, Lennie holds her so tightly he breaks her neck.