Cracks in tile are usually a symptom of something else going on below the surface. When installing new tile, I try to make sure that everything is put together so the tile looks great permanently. (See this article) Unfortunately not every tile is installed with such care, as was the case with this project.
The homeowner had been dealing with the large crack down the middle of her bathroom floor for a long time. It bugged her, but she put up with it until she was ready to remodel the entire bathroom.
The most likely reasons that tiles are cracked or loose are:
1. Not enough mortar underneath the tile for support
2. The wrong type of adhesive or mortar was used
3. Something below the tile is loose such as subflooring or framing
4. Moisture has damaged the subflooring or walls below and loosened up the mortar
In this case, I suspected it had to do with something under the tile and how it was attached to the subflooring. I was right.
The crack was directly above the joint of two pieces of cementboard below. The cementboard was not installed correctly and was loose. Cementboard has to be installed over a bed of mortar and nail or. screwed down. This cementboard had no mortar underneath. This meant that it was not thoroughly supported and moved just enough that the tile cracked at the joint over time.
I see this mistake with cementboard fairly often. Whatever products you use, it has to be installed according to the manufacturers instructions so that it holds up over time and works as it is designed.