Thanks Akira, it was a lucky consequence of a freeze-up of my cabin (dry cabin, no pipes to get compromised). I think what happened is the following: The very clean water got supercooled at the inside air temperature of -7°C. When I came back I must then have turned the bottle upside down, at which point the water immediately froze while the air was rising towards the bottom which now was at the top. The interaction between the walls of the bottle likely caused the friction that seeded the freezing, thus the bubbles could rise in the center.