Too much water in the grout when it was mixed. Needs to be redone.

Hi hollysprings, we wondered about that too. But it's not crumbling anywhere between the tiles. Only where the grout interfaces with a wall or with a cabinet side. If it was too much water, wouldn't we see crumbling away everywhere?

Anywhere that a tiled surface meets some other surface, you need an expansion joint. This is because different materials on different planes will have a slight amount of movement due to humidity changes and/or just use. Grout is an inflexible material and it will crack and crumble as the surfaces move together and apart (even a hair).

The only fix is to clean it out and redo the joint with caulk. You can get sand-textured caulk in a color to match your grout.

Yes, there should be no grout there, only flexible caulk.