Basically, very few theories account for how many countries are having it and what is the common factor, even part of the Islamic countries and so on.
The previous best theory was integration of women in the workforce in typically male jobs (jobs that have flexible hours and can be done along with children historically didn't affect fertility).

But I came up with a better theory: Lost knowledge from urban displacement. When youth either move away from (for college or jobs) or loses their elders in war, the chain of transmission of the knowledge of how to be husbands, wives, raise children, cook, etc.. is lost and has to be relearned. Displacement is very universal in human recent history.

The glamourized office/take out food/prebuilt everything/rent everything lifestyle is very specifically low inherited knowledge based, so I purport that the youth is not choosing it because they think its better than the older lifestyles, they simply suck at any other life style and could not take care of themselves otherwise and have nowhere to seek to learn those skills.