Mostly, it's lack of light.
Beeing used to outside light, they pop out 1 or 2 leaves when brought to the warmer inside, using stored energy. When the naner eventually runs out of energy, there is no new energy source (in this case light) to keep it going. In my opinion, dry air is not the main concern, low light levels are.
I solved this problem by using a high power natrium lamp. Now my biggest concern is keeping the plants contained until next spring!
Erlend