My iphone 5's screen is broken after coming back from mail in service...What happened?

I just got my iPhone back from mail in service through applecare and the screen is shattered! I refused service because they wanted me to pay for it and so they sent it back. I originaly sent it for them to fix the microphone because it stopped working, and then i got an email saying they found accidental damage. There was a picture on the support page but it wouldnt load. So what the heck happened to my iphone because I know I didnt send it like this.