The one reason, and most common, is that the installer watered the grout down too much. He must have thought that it was easier to apply, which it was. But it made for weal grout and now it is cracking and chipping out and you have gaps in your tile floor. This is an easy fix. Go to the tile store and get a diamond shaped grout remover. Also, get a linoleum knife. The linoleum knife is used to chip out the deep grout and the grout remover will clean up the edges.
Once the grout is removed, mix and install new grout. Closely follow the instructions on the package. You just repaired your own grout for very few dollars.
If your grout is cracking and chipping out for the other reason, you will have to remove your tile and retile. This time make sure there is quarter inch thick cement fiber backer board laid under your tile. You might want to call a professional.