Why does the wine cap float when fermenting?

Because in the process of fermentation, the anaerobic respiration of fermentation bacteria produces carbon dioxide, which increases the pressure in the fermentation bottle and makes the wine lid float.

"Wine cap", also known as bottle seal and bottle cap, is a small thing similar to a hat on a wine bottle. The bottle mouth used to wrap wine when sealing.

For wines that do not need long-term storage or almost no ventilation, sealing with aluminum screw cap can play a dual role in preventing wine from flowing out and wrapping the bottle mouth at the same time.