What all can cause molting?

I think it is not only due to their age, but to day length too.

 

I've always thought that even with artificial lights a hen has to molt sometimes.
What I'd like to know is how long it takes before they start laying again? I've got three hens and a roo, two white ones and one red(had two reds, but lost one to a hawk). They are under a flourescent light (14hrs) and our winter has been mild, only a few nights below -25C.
Back in early Dec the two white ones started dropping feathers and have only given one egg in the past 30 days. The red one still gives 6 eggs/week. They still look fully fledged, but there are lotsa white feathers on the coop floor.
How long does this go on? Before the molt started we were getting 2-3 eggs/day, since we got them (used) last spring.

 

And to supplement Dawn's post.

Kansas State feather loss
http://www.ksre.ksu.edu/library/lvstk2/mf2308.pdf

Stress was not mentioned in the article, but it can cause a molt. Instead of full molts, these are often partial or mini-molts. Something that often causes unseasonal molts is them running out of water for a while. I even had an Australorp that hatched chicks in late August go into molt while she was raising the chicks. She had finished her molt and was back to laying eggs before some of the others started molting. It was nice getting one large adult egg a day when the others shut down.

Pullets less than a year old when fall hits do not always molt. I've had some start laying in late September and not molt until the following year, laying the whole time.