Back phone glass cracked

It isn't taking the case off that usually does it, it is putting it in the case that does. If you have AppleCare + they can repair the back screen for $29. If not the price is shown under "other" repair here -> iPhone Repair - Official Apple Support

You either hit something with it, you carry it in your back pocket and sat on it thus putting too much stress on it, or you put too put too much stress on it when you put it in the case (this happens with the Otterbox cases). Click on the Get Support link at the top right of this page to start the service process.

Click on the blue letters in my last post.