Sunday morning found a pool of sticky liquid on the floor, clear in color, no distinguishable aroma. It did not take long to trace it to bottle of dessert wine on the nearby wine shelf. The cork was hanging by a shred of the foil. After clean-up, we examined the bottle. The remaining wine was cloudy. It smelled alcoholic and sweet, quite normally. We used a coffee filter. The result was quite normal and drinkable.
I theorized that the fermentation was still going when we bought the wine, only slowly. The liquid in the bottle sealed it air-tight. The increased gas, from fermentation, pushed the cork, against only the foil and the friction. We knew which side won.
Normal dry wines, even Champagnes, left no sugar for fermentation. The system is stable if under reasonable ambient temperature. This is really only possible with sweet wines.
Lesson learned? Drink your dessert wine quickly, or store them vertically.