Mammele,
It is a Chassidishe Minhag.
I once asked someone why Chasidishe women shave their hair, he told me it is a Zohar. The Zohar states that even if the hair is covered it should be cut.
He showed me that the Mishne Berura references this zohar. I do not remember where it is, however if this is important to you, I can try looking it up.