There is no set time to molt. The most common time is in the fall or winter when the days get shorter. The shorter days is their sign that it is time to quit laying and spend that energy growing new feathers. If you have been keeping lights on but changed that recently, it could cause it. But stress can cause a molt too. A commom one is running out of water. But anything that causes stress might cause them to do a partial or even a full molt any time of year.
It is also not unusual for young chickens to totally skip the molt their first fall/winter. The pullets keep laying throughout the winter and don't molt until the following fall. They can get pretty rough looking with those worn out feathers.