Microplastics causing infertility is the main reason for that. Hard to have babies when men's nuts can't properly produce viable sperm or women can't properly form a placenta and their ovaries have atrophied.
There are also socioeconomic reasons where people are avoiding pregnancy, plus the breaking down of community togetherness exacerbating the "loneliness epidemic" and people just aren't meeting each other and going on dates anymore, also due to socioeconomic factors. There is also the antinatalist movement but unsure as to the size of it.
Either way, the call to "reduce populations" is a bullshit argument. Just because it is happening naturally (or due to natural phenomena as a result of the externalities from human activity) doesn't excuse the call for an authority to dictate that decision for others.