Women, regardless of their previous type become ISFJ upon motherhood...or at least ISFJ-like.
ISFJ qualities create the upright protective mom who combines Si's need for the comfort and security of tradition, with Fe's natural empathy, caring and organizing. They are the stereotypical home-makers.
This is based on unreliable anecdotal evidence, but I think a combination of instincts, and society's expectations play a role in this.
My mom's an ISFJ, and from reading through threads on this topic, it sure seems like there's a ton of ISFJ moms out there.
I don't think they're all ISFJ's, they either naturally fall into that role, or feel that they need to become that way due to societal pressure.