wall tiles cracking after install

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.

Movement of the tiles due to temperature changes.

See [ My wall tiles seem to have hairline cracks in the glaze ] for example.

Did the cracks run parallel with a joint in the substrate? I'm guessing it's a partition. Sometimes this is caused by shrinkage/ expansion in the timbers as mentioned above

Hi,
From time to time, small, hairline cracks can appear in the glazed surface of tiles. This phenomenon is referred to as 'crazing.' Tiles are not designed to craze, but it is a recognised inherent property that can occur with glazed tiles. A crazed tile will still perform properly and there is no reason for concern. Crazing can often be seen in old or antique pottery or chinaware.
Continuous movement of a substrate or structural wall can cause this phenomenon.
So what can you do now?
A crazed surface will not affect the performance of the tile. Generally, most people will simply ignore the issue.
If your project seems to have an extreme case, or it is personally annoying, then the only way to resolve it is the replace the tiles.
The cost to replace crazed tiles can't be charged back to anyone in particular.