As with so many of my favorites, I saw the movie before I read the book. I came away wondering, "Why is it called The Postman Always Rings Twice?" There is no postman in the story (book or movie). We don't hear anyone ring twice, and no one mentions a postman ringing twice.
Wikipedia has an interesting article on this title. In the preface to another of his classics, Double Indemnity, James M. Cain said this title came from a conversation with screenwriter Vincent Lawrence, who grew so anxious about his manuscripts being returned he would leave the house when the postman was due, causing the postman to ring twice.
That may explain the origin of the title, but it doesn't explain why Cain adopted it as the title of the book.
The article suggests three other possible explanations for the title. I don't find any of them persuasive because each would assume the reader (or viewer) had some specialized knowledge of events or ideas not contained in the story. That's a lot to ask.
For me, the title works because it is senseless. Like naturalistic classics by Jack London, Upton Sinclair, Richard Wright, and others, noir stories suggest our actions are determined by our circumstances. If so, what we do has no more meaning than the wind blowing or a river running.
Why should a title make sense?