@fu I'd say there's definitely overpopulation. But for each individual nation, reversing population growth brings the potential hazard of too many retirees for the work-age population to support.
As far as I know, this hasn't happened in any nation yet, but the prospect scares political leaders (and especially nationalists, who want to fill the world with people of their nationality and loyal to their government)
Also, if you're Russia and you've been preparing to start wars of reconquest since 2008 or so, the possibility of population growth soon becoming negative necessarily means that your war has to occur sooner rather than later.