This is a replacement phone, when I received it there was no damaged. I have not dropped my phone, I noticed today that the rear screen has several cracks. Yes, it is in a case. I don't understand.
Rear screen? I'm gonna assume you're referring to the glass back.
Pressure on it could have broke it, leaning on something while it's in your pocket/sitting on it/setting something on it. An extreme temperature change can cause glass to crack as well.
Welcome to Android Central! Unfortunately, not much we can help with you about that. This is one of the downsides of the silly obsession with "premium build" glass backs. Glass breaks.:-\