Most likely over-filling, but could also just be that your "pulling" (drawing) too hard and forcing the juice into the air passages.
If this happens, take the cart off the atomizer, grab a tissue, cover the opening (stuffing end) with a finger tightly, and blow hard through the mouthpiece.
You will see along the sides of the cartridge, that slips into the atomizer, now needs wiping, do again...... and again if necessary, and wipe it dry.
If its filled quite a bit, you can also take a paperclip and put it in the small hole of the mouthpiece and pry the end cap off. Wipe up any juice sitting there in that area, replace the end cap and carry on.
You will find this may be required occassionally when those passages get juice in them, and is really a part of "maintenance" should you also refill your cart and do the required washing/sanitizing of the cart housing.
Also, as a hint when place a full cart onto the atomizer, make sure you don't cover the mouthpiece hole will your finger as you press it on since the pressure built up by doing so will force the juice in the atomizer (possibly flooding it), and those airway passages.
Hope this helped....
VapeOn,
Greg