I guess I do not know enough about South Korean fertility to make a credible claim, but if most of the fertility decline is due to birthing children at older ages, then I would file that under the same category as women gaining financial independence and ability to choose when to have kids.

Women basically saying in order for me to risk what I have to risk to have a child(ren), I need <x> security first. Which can manifest as working to save and achieve higher income levels instead of having kids.

But I suppose there could be a huge environmental or whatever biological factor causing reduced pregnancies. Marriage rates might help tease this out.

Edit: from 2019

https://www.straitstimes.com/asia/east-asia/til-death-do-i-s...

> A decade ago, almost 47 per cent of single and never-married Korean women said they thought marriage was necessary.

>Last year, that fell to 22.4 per cent, while the number of couples getting hitched slumped to 257,600 - down from 434,900 in 1996.