What happened to Curley's wife? Why did this happen?

Curley's wife is terribly lonely. She seeks companionship with the only person who will talk to her. She finds Lennie with his dead puppy in the barn. Lennie has smothered his puppy to death. Curley's wife allows Lennie to stroke her hair and things go downhill from there. Curley's wife panics and Lennie panics, inadvertently killing her.