What is causing these lines?

So I recently had the screen of my phone replaced at a third party repair store. It cost me just under 200 Australian dollars. The day after I noticed I couldn’t press the p button without struggling for a while and other buttons on the very edge had issues as well. So I headed back to the store and he happily fixed it with no extra charge. But the next day I noticed a thick vertical line on my screen and subsequent horizontal lines all connecting on a small crack. The crack is definitely not on the glass and is an lcd issue. So once more I head back to the store and the guy claimed that the damage was caused by squeezing of the phone. I use a thick tyre rubber case that would make it very difficult to do so. Not to mention the glass in perfect condition which would probably break if squeezed hard enough to break the lcd. My question is 1 what actually caused these lines? And 2 is there any way I can prove the damage was caused by him and get a repair? Thanks in advance for any help.