Crystals formed in my wine bottle, why?

The crystals you see are potassium bitartrate. These will not actually hurt the wine, but are unsightly and a nuisance to get out of the bottles once emptied. This is excess tartaric acid that is precipitating out as a more complex compound.

Potassium in the grape combines with tartaric acid to form a sparingly soluble potassium bitartrate. The solubility is reduced further by increased alcohol and the result is precipitation of crystals.

Tartaric acid has two available H+ ions and the potassium ion replaces one of these. Thus, the potassium bitartrate is still acidic and is actually lowering the acid of the wine surrounding it. If you chill the wine for a week or so, even more crystals will form.

You have two choices

1. Open the bottles and very gently transfer the wine to new bottles. Chill the wine first for a week or two to encourage as much of the excess tartaric acid to precipitate out as potassium bitartrate. Transfer the wine with as little disturbance to it as you can manage, as at this point the risk of the wine taking up oxygen and oxidizing is very high. OR

2. Leave the wine alone and decant it before drinking.

To clean up the bottles with the deposits in them, fill them with very hot water and let them soak for a couple of hours. The crystals should dissolve, but a bottle brush may be necessary to aid the process.

To prevent this from happening next time, cold stabilize the wine for two weeks or so before bottling it. Wine does not freeze until it reaches about 25 °F., so you can actually bring the wine to a 30-degree chill without worrying about it freezing up in the carboy.