Shrinkage, is what it is. And without a cut joint when they installed the concrete floor, it may happen somewhere else.
Welcome to the nature of concrete. This is typical installation unless you get a really good contractor.
You could have someone cut a line down into the concerete at one of your grout lines, this will give it a control joint along which to crack in the future. Then you are just fixing grout.
Hope this helps someone who will be installing tile over concrete, and