18 months is typically the first molt for a lot of breeds. Because they are normally hatched in the spring, they generally molt in the second fall of their lifespan. Have you noticed excess feathers laying around the coop, or somewhat ratty looking appearances on your birds? Since the number is so large, this cannot be an egg tract issue (i.e. egg binding or calcium problems) and I would most likely attribute it to the declining daylight and colder temps. Hopefully this helps.
Also, appps could be correct in saying that it is just a phase your flock is going through, so I wouldn't discount it, but a good number of birds in my flock just began molting, so I am more inclined to say that is the cause.
