Depends where the grapes are from, what species they are and of course also the winemaking the process. There was a joke going around about the difference between German wine and vinegar: the label.
In sunnier regions the grapes are sweet but don't get enough acidity and the winemakers have to compensate using tartaric (also citric or malic) acid or ideally more acidic grape juice (imported or from other grape species). In less sunnier regions the grapes don't get enough sugar and they become too acidic. Fermentation naturally stops at about 13% alcohol content but if it's warm in your cellar (>18°C) it can start again and exhaust all the sugar in the wine. When this happens, you can stop the fermentation by adding suplhur. A small amount of sugar is necessary to preserve the flavours in a wine if it's kept in barrels or stainless steel drums. Commercial wines are filtered, pasteurized and bottled after fermentation stops if they're not aged in wooden barrels. Once a bottle of regular (unaged) commercial wine is open, the flavour goes away if left overnight.
In sunnier regions the grapes are sweet but don't get enough acidity and the winemakers have to compensate using tartaric (also citric or malic) acid or ideally more acidic grape juice (imported or from other grape species). In less sunnier regions the grapes don't get enough sugar and they become too acidic. Fermentation naturally stops at about 13% alcohol content but if it's warm in your cellar (>18°C) it can start again and exhaust all the sugar in the wine. When this happens, you can stop the fermentation by adding suplhur. A small amount of sugar is necessary to preserve the flavours in a wine if it's kept in barrels or stainless steel drums. Commercial wines are filtered, pasteurized and bottled after fermentation stops if they're not aged in wooden barrels. Once a bottle of regular (unaged) commercial wine is open, the flavour goes away if left overnight.