Historically, IIRC (it's been ages since I read up on this) there was something of a split, with one major strand of radical feminism becoming very biologically-essentialist about gender even though that's in total contradiction to some of the other ideas (gender as wholly a social construct, etc.). So this is not coming out of nowhere or devoid of ties to older traditions of radical feminism. There were definitely bouts of this sort of stuff in the late '70s/80s etc.

Even though most of the current TERFs are not in any sense "radical" (or, in many cases, "feminist").