I noticed that mineral water with high mineralization doesn't have salt flakes floating on the bottom of the bottle on store shelves, but when I open one, after some time they start accumulating. Does it have something to do with decarbonation?
Salt solubility decreases with decreasing pressure.
It could be gases (oxygen or carbon dioxide) from the air, competing as solvents with the salts. I know the reverse effect: adding salt tends to drive out gases.
I'm not sure whether this gas explanation is more significant than the pressure explanation. But, you could do an experiment: briefly let the pressure out and then retighten the cap without letting fresh air enter, and see if the crystals still form.