I just ran across a post from a #librarian on becoming a systems librarian: https://www.hughrundle.net/2017/10/29/how-i-ended-up-here-librarian/
Dumb luck and fear definitely played a role for me too (though not the patriarchy bit). I got an undergrad degree in computer science because it was the fastest way for me to graduate so I could get my MLS. I started at the reference desk but applied for a systems job because the library was discussing reducing the reference staff. Systems librarianship turned out to be the perfect job for me.