Just a gut feeling, but I have felt since they built N3 that Mom didn't really want to move. She loves the hatchery area and isn't bothered by people, unlike DM2 who is very shy or just plain bothered by humans being too close. I think he wanted the new nest away from us humans and the street traffic, and because her natural instincts are to procreate, she went along with it. It will be interesting to see where they nest next season.