Inside glass panel on my iPhone 4 smashed?

I've had my iPhone 4 since Christmas and I have it in my pocket all the time and it has never smashed! I have a plastic case on it as well. Yesterday I got it out of my pocket and the inside panel on the back had smashed but not the case or outer panel on the back! So it's not sharp to run your finger over, but I don't understand how this could've happened? As the case and outer panel didn't smash, but the inner one did? Any ideas on how this might've happened and how much it will cost to repair?

$20 bucks on amazon for the parts and tool kit, probably $60-75 at the repair store with installation.

http://www.amazon.com/...007WUMQEO/

Perhaps posting this question in the consumer electronics section may get the answer you seek