You're very welcome!
They were indeed unusual at the time. I have no idea what exactly brought about the sudden change in dress. I would suppose that these outfits were for casual, every day outings.
I remember reading that even the peasants the girls met during stops, even peasants that had children the girls played with, were shocked at how they dressed. If I remember correctly, they stated that not even peasants would wear such clothing.
Of course, they always dressed formally for the fitting occasions, but they wore these outfits on the Standart, at Stavka, and around Tsarskoe Selo.