Original Leather Case made my iPhone 13 pro back glass cracked

That phone sustained a hard fall as there is no way at all a leather case could have caused such damage to the back of your iPhone. If the case could cause such damage, we'd see this forum littered with threads from people saying the same and yet, here we are on the worlds largest forum about iPhone and there are no such posts. As you've learned, user caused damage is not covered by Apple warranty.

Hello ~ You can clearly see on the back glass that something impacted your iPhone. The cracks emanate from the one side. If the case itself were defective there would have been numerous reports by now. Your case also shows quite a bit of wear…are you carrying it in a pocket perhaps?

~Katana-San~

It is much more likely that at some point since you put the case on it, the phone was dropped or suffered an impact that caused the glass to break, and you just didn’t notice it until you removed the case.