I'd assume environmental, but there's also more subtle answers than will fit in a comment box — whatever the cause, it has to be near-global.
China's building loads more houses, still has a fertility decline.
It is really a good example of what natural dimension reducers we are, even when we know it makes no sense. It is like we can't but help ourselves to reduce things to one explanatory variable.
My favorite is the news headline "The market went up today because of X".
They say: Tesla shares up as revealations surface that the wind is blowing east.