The grout in my main shower has developed cracks. They started small but over time they have grown and there are actual crevices in the grout. I've been using my guest shower so as to not cause mold from water penetration, though it's possible there could be some underneath since I was using it for quite a while. 

So basically, I want to know the proper way to fix the cracks and why they begun in the first place. Was the wrong type of grout used by the person who remodeled the shower? I can describe the grout as a very rough grit and brittle. Should I re-grout right over the old grout with the proper grout? What exactly is the proper grout for a tiled shower? Would it be easier to just touch it up with caulk of a similar color? 

This is all assuming a complete re-do of the tile is not something I'd like to do at this moment in time, maybe in the next few years though. If you do feel that the possibility of mold underneath justifies a complete re-do right now, please elaborate on that if you can so I may consider it. There is no visible mold anywhere and I wouldn't suspect any other than the fact I know there are cracks and water must have penetrated for some length of time. Thanks for any advice.

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  [1]: https://i.sstatic.net/cicVy.jpg