There could be several reasons why your subway tile is cracking. Most often, cracked subway tiles result from improper installation or the tile supporting more weight than it can bear. If that's not the reason, you could have an issue with the damaged substrate (the wall or floor surface under the tile). It could also be a matter of the tile being exposed to extreme changes in temperature, which loosens the grouting and adhesive.  Lastly, if you drop something heavy onto your tile or strike it with a hard object, the tiles can crack.