Possible explanation:

The floor is equipped with floor heating. There are pipes inside the screed of the floor that are connected to the heating system. I assume that the floor has been constructed recently, and that the picture shows the result of the first usage of the floor heating, while the floor was installed too hastily.

What could have happened here is that the screed floor with the floor heating pipes has not been let dry enough before it was covered with floor tiles. When installiing a screed floor there is a lot of moisture from the screed present. It needs to evaporate first. It usually is done by running the floor heating for several weeks without having the floor tiles installed yet. This allows the remaining moisture to evaporate through the screed. The tiles usually are installed when the screed has been dried enough.

When the floor is covered with tiles without letting it dry properly first, the tiles seal the floor and remaining moisture cannot evaporate. When the floor heating is in operation, the trapped water vaporises and can't escape the floor. Due to the pressure that is being created, the floor bursts open violently. Compare it to water that is being heated in a pressure cooker and it can't escape.