I'm not well versed in the causes (and am curious what sources people may have on them), but I do want to say, a curious part of any conversation about birth rates to me is how common the assumption is that "falling birth rates = bad". On reflection, I find it to be kind of odd because we're living on a planet with billions of people. I do sort of get that logistically, you don't want to have a country with a disproportionately aging population and not enough youth to take over the roles they were doing. But also, if a society is organized more around need than expansion, it should be somewhat feasible to pull back on what is built out and so on, especially with the increasing capability of automated tools.
I don't know, maybe there's something I'm missing, some factor as to why it'd "of course be a bad thing". But although I don't believe in the "overpopulation" narrative as a climate change issue, I also don't see how birth rates staying the same / going up is inherently a good thing. My most pressing concern is the people who are alive and what world they have to deal with, and so much of the pain in that has so much to do with where resources are going rather than how many people are alive to do the labor. Like in countries where resources are going to military rather than public services, is it even worth talking about birth rates? What difference does it make (toward a humane, sustainable society) how many are being born if resources are not going to people's needs anyway? I don't mean that as a shutdown of talking about it; just questioning where the narrative on it typically arises from and for whose interests.
But I would like to hear from people who know more on the subject.