Grout between kitchen floor tiles can loosen for many reasons. The grout will crack, break up and eventually cause tiles to shift.
Faulty Sub Floor
If your sub floor, that is the wood the tile is laid on, is loose or gives, it can cause the grout to crack and also possibly the tiles to break up.
Old Grout
Most people don't realize that not all grout jobs will last for the life of the floor. Very often they have to be scraped up and replaced. Sealing the grout will help keep this job to a minimum.
Improperly Mixed Grout
Grout that was mixed with too much or too little water will tend to disintegrate and fall apart. It can actually turn to a sand-like material and be swept up when you clean the floor.
Source: www.DoItYourself.com
