The cracks could also be the result of an improper underlayment as well as possible foundation problems.
I would suspect that if there were foundation problems, you'd see other signs as well. Cracks in the wall, unusual settling outside the home along the crack line, etc., etc.. And the cracks, IMO, would show bit by bit over time. If they showed up all of a sudden, and the problem was foundation-related, then you better get out of there!
My first suspicion causes me to ask what layers of flooring are under the tile from the joists/concrete on up. Also, did someone happen to wheel an extremely heavy 'item' across your tile floor recently w/o first putting down a sheet of plywood or something? One side of the cart over-top of one wheel may have had a great deal of weight and, thus, cracked the tile. The only other thing is the thickness of the tile is extremely thin or from a bad batch. More unlikely but worth considering. FWIW, my neighbor installed tile over-top of his kitchen floor w/o the proper underlayment. His floor 'bounced' a great deal (when it comes to having a tile floor) that 2 years later, he ripped it out because of the cracks in the grout AND the tile itself. Just some things to think about.