I was wondering if this is due to our foundation cracking.

One way or the other, it's due to movement in the structure not being isolated enough from the tile floor. If your foundation is cracking, I'd say your tile floor is the least of your worries. But yes, it could cause the tile and/ or grout to crack. It could also be one of several more subtle causes, such as not putting any thinset under the backerboard when installing it, using the wrong thinset, etc., etc., etc..

Is this a sign of more cracks to come? Should this be repaired and if so how does one repair this?

It may or may not be. It would take a physical inspection to determine for certain exactly what's causing the movement. That would also be required to repair it, because before you repair the crack you need to determine what's causing it, or else it'll just keep coming back.

Our contractor added another layer of grout. Did the second layer of grout permanently bond with the first layer?

That depends. How long after the original installation did he add the second layer of grout, and approximately how deep were the joints prior to the second layer?

Thanks for your reply Bill. You have always been there to answer my questions. Thank you for your expert advice.

First off the tile was installed right onto the concrete slab. No backerboard, just thinset. I remember seeing the crack in the cement (which we were told was normal) foundation.

The second layer of grout was added about two weeks after the first layer. As for the depth, I'm not sure. The tile is not rounded but squared off and when I walked on it it would pinch my feet.

The bathroom floor flows into the shower. There is no step over. I guess this is called a handicap entrance. We have the frameless glass. The glass was installed and then the second layer of grout was added. Where the glass meets the floor tile there is silicone but mold is accumulating under the silicone in the grout lines where the second layer of grout could not be added. Can this be corrected?

Thank you again for your feedback.