It is a prevalent practice in Satmar communities, and possibly other Chassidic groups.
I am not certain what does shaving mean, whether it’s down to the scalp or say level 2 on a shaver, etc…
I read that it’s mekor is a Takana of the Vaad Arbaa Kehilot a few hundered years ago in Eastern Europe and that the Takana is still in force today. The Sanzer Rav wrote extensively (I believe) on this subject.
As noted by other posters, it seems to assist in Tevila since we are makpid about hair knots, chatzitzas, etc…
The Zohar speaks about hair in general and the severity of exposing it.
Shaving one’s hair could explain a Gemara that Kimhi merited to have 7 sons be Cohanim Gedolim in the Beis HaMikdash. The Gemara makes an observation that the walls of her house never saw her hair. That being so, there must have been instances when she would have to remove her hair covering; i.e., bathing, etc… If she did shave her hair, then naturally, the walls could not have seen her hair, even if the hair covering, was momentarily removed.