Could be a lot of things.
Sometimes there are Very, VERY Small Air Bubbles suspended in
e-liquid. And they would like to Group Up with each other but they don't have anything nice to Stick to. So they are Happy to float around in you e-Liquid Bottle. But once they see a Wick, thy say, "Cool, there's a Nice Place to Park and take a Rest".
Other time it could be the Air that goes into the Tank to Fill the Vacuum that the Vaporized e-Liquid causes. If something goes Out, something has to go In.
Or it might just be Sun Spots?
Hard to Tell.