In November I had a tiler do some work in my batroom. Last month I noticed that a few of the tiles in the shower area had hairline cracks so I asked the tiler to come back and swap them out. He did this but was puzzled as to what had caused this. Anyway, I have now noticed another tile, not in the same location, also has a crack. I'm 99% sure these tiles did not have cracks when installed, the tiler would have noticed, and was wondering if anyone had any idea what might be happening here.