Your substrate is flexing. Could be rot, could be separation to thinset, backer failure or expansion/contraction of the house. Access the subfloor underneath and make sure it’s not rotten. If not, cut out the grout and redo the whole floor. Patching will look like shit, never match and just crack again. If it’s rotten, get out your checkbook, bathrooms ain’t cheap and you’ll end up doing the whole thing.