Your tiles, and keeping them looking beautiful, is a huge part of keeping your bathroom looking shiny and clean. Cracked grout can really let a renovated bathroom down, especially when it gets dirty, because cracks can catch and retain more dirt than a smooth surface.
When the water in the grout mix evaporates, it leaves minute holes. But when there is excess water, the tiny holes will be larger than they should be and the grout will be prone to cracking.
Thinset is the glue that's put over the mortar. When it dries, the tile's pulled closer to the mortar. But when it's not thick enough, gaps will be leaving gaps of air under the tile (and sometimes you'll hear that crunchy noise behind a tile that indicates cracking grout).
Changes and movement in the substructure can cause cracked plaster, and that stands to reason that tiles and grout can crack too. You should be able to identify this as you will notice other areas affected too.
If grout isn't forced into the gap between the tiles hard enough, there will be a gap between the grout and the wall which ensures that the grout is subject to easily breaking due to any pressure.
If water's added to the cement mix too late (after the grout begins to cure) then the grout will be prone to crumbling and will be weak.
Many of the causes of crumbling grout can be attributed to poor tiling technique. It's essential to install your tiles professionally and avoid these mistakes undermining the beat of your tiles.