It is more common on the 3 piece attys since the cart can be refilled easier as it is made to be refilled.
The most common cause of liquid in the mouth is by the following causes:
Holding the ecig not at a 30 degree down angle when
vaping, ie straight out or up.
Overfilling the 3 or 2 part ecigs (yes cartomizers can be refilled and overfilled).
Leaving the egic resting in a mouth piece downward angle or carried in a in flat position during transport.
Hope this helps
Dan