The most common reason for this is a build up of condensation or vape liquid in the centre tube. It can be cleared quickly by removing the mouth tip and flicking the device sharply downward a few times. When working with a brand new coil or newly primed coil, you may find you have to do this a couple of times before it settles down.
Leaking can happen is if the coil isn’t screwed in all the way, and liquid is seeping down through the base into the airflow ring.
To solve this, take the tank off the battery and turn it upside down. Then, unscrew the base of the tank from the rest of the tank and check if the coil is screwed firmly into the base. If not, remove and then refit the coil, tightening it to just finger tight.
If the problem persists, change the coil, as an older coil can also lead to leakage as the cotton breaks down with use. However, first make sure all the liquid is out of the base with a paper towel or by rinsing the base and leaving it to dry thoroughly.