There are too many reasons to guess at it over the Internet. However, the two most likely causes are 1) the house is shifting more than anticipated, creating a "fault line" of sorts where the cracked tiles are located; 2) poor installation (shoddy underlayment with plywood instead of concrete or cement board).

Other possible causes could include the following: Some sort of moisture is creeping from underneath so as to weaken the bond between the tiles and floor underneath, perhaps due to a slow leak; an un-level underlayment, creating a peak underneath a row or two of tiles.