These two links give some good information on molting and what causes it. There are even a few sketches! What could be better?
Mississippi State describes molting
http://msucares.com/poultry/management/poultry_feathers.html
Kansas State feather loss
http://www.ksre.ksu.edu/library/lvstk2/mf2308.pdf
Chicks will go through two juvenile molts before they reach maturity. They grow so fast they outgrow their feathers and need to replace them. I usually can't tell when they are going through a juvenile molt by looking at them, but there are suddenly feathers everywhere. It varies some but mine usually have their first juvenile molt around 9 weeks and the second around 14 weeks.
Many times chickens will not have their first adult molt the first fall. Pullets often skip the molt the first fall and keep laying throughout the winter, not molting until the nest fall. But a few do. Many first time chicken owners see their pullets laying through their first winter and think this is the norm for every winter. It is not the norm. They'll go through a molt the following fall/winter.
Many different things can cause a molt. The normal one is that when the days get shorter, they shut down egg production and they molt. Instead of laying eggs and trying to raise chicks in the bad weather, they use the energy that was going into egg production and raising chicks to grow new feathers instead.
But the days getting shorter is not all that can cause a molt. Stress can start one. It may not be as complete as the normal fall molt, but it is still a molt and egg production can drop or stop. One common thing that starts a molt is to run out of water. I don't mean run out of water for a few minutes, but long enough to really weaken their bodies. Even the stress of a predator attack or moving to a new location can start a minimolt. Anything that causes stress.
Hope this helps. Good luck!
