^^This^^
With any tank, overfilling will cause spit-back, leaking, or flooding...
a leaking coil (improper wicking) will cause juice to build up in the tube and/or run out the air-holes...
A poorly sealing tank (chipped glass, bad o-ring) will cause the tank to lose negative pressure (vacuum seal), which will allow juice to leak into the center tube or air-holes...
A coil not fully seated properly into the base or attached to the center tube will work the same to lose negative pressure, or create an all-out leak...
Using too low of a Wattage/Voltage for your coil will cause juice to not fully vaporize, resulting in spit-back, popping, or liquid remaining in the tube...
This last one is the usual suspect