No knowledge on material
Not follow procedure.
Use wrong material.

Most tiles need to soak in water adequately so that it can adhere to normal cement properly. Otherwise this happened. Some contractor just dipped momentarily or not at all to save time.

Nowadays, there are mortar specifically for tiling. This can eliminate the need of soaking but will secure the tile properly. However, most likely that the contractor will not use because either they don't know or just to reduce cost.

I asked if they can just put tiles on top of other tiles without hacking. They said cannot. It will popped up later, but my toilet I just put a thin layer of epoxy meant for tilling and it has been 10 years without problem.