Also, have you considered worms to be a possibility?
Personally I would suspect loss of the other hen might affect them, Maybe the addition of the pullets could affect them too.
By the time they were over it, they might have begun to molt or it may have been unusually hot. There are a lot of...
How far north are you? I'm on the 45th parallel. Mine slowed down at the end of August, currently only my two under 1 year are laying. The rest are 3 1/2.