I noticed these bubbles in my first bottled batch of Witbier.
My understanding is that when we bottle carb the bottling sugar we add feeds what few yeast are left in suspension and this releases CO2. The CO2 then rises into the head space in the neck of the bottle. When there is no more room in the neck for the CO2 it begins to go back into the beer and becomes suspended in the solution.
I'm not a chemist by any means, but this is what I have read. Perhaps someone has another/a better explanation?