Cartotanks operate on the negative pressure principle. The tank itself is a liquid/air sealed container. When one draws on the cartomizer while vaping, the suction causes e-liquid from the tank to replace the e-liquid that is vaporized in the cartomizer, resulting in air replacing the lost e-liquid in the tank.
Cheap quality cartotanks may not have as tightly sealed tanks resulting in air leaks upsetting the negative pressure in the tank - resulting in the carto flooding. The same can be seen with many of the tanks that use a side or top juice fill hole. Many of the problems of the Smok U-DCT and AGR cartotanks can be attributed to this and is why I never recommend them.