If you're curious, here's the technical bit. The coil is a small heating element wrapped in cotton. When the e-liquid is thin, it saturates the cotton faster than the coil can vaporize it. The excess liquid sits on the coil and in the airflow channel, and when you draw, it bubbles and gurgles. In severe cases, liquid can reach the battery contacts, which is a short-circuit risk. That's why persistent gurgling isn't just a nuisance—it's a reason to stop using the device.

Some vapers try to fix a flooded coil by blowing through the device to push liquid out. I'd advise against that, because it can push liquid into the battery area. Stick to the gentle fixes above.