The increase of male births can be linked to a historical preference for sons, a decline in birth rates driven by gender selection preferences and advancements in technology, such as ultrasound, that facilitate gender selection, according to the report.    The report also emphasized that the prolonged period of a skewed sex ratio at birth since the 1970s has had a significant impact on marriage rates. Regions with high marriage rate imbalances are characterized by particularly skewed sex ratios at birth during the 1980s and 1990s.

So basically a lot of people saw a vagina in the ultrasound and decided quit and restart the level.

I assume this increase in men being single has led to increases in male loneliness and subsequently the massive chauvinist backlash and rightward shift that's been happening in S. Korea.

Since the 1970s, the sex ratio at birth in Korea has exceeded the natural ratio — 104 to 107 boys for every 100 girls.

The short explanation

Interesting in that, flipping it around, there doesn't appear to be another country with as large a disparity.

Armenia is #1 with 55% women:

https://www.worldatlas.com/society/10-countries-where-women-far-outnumber-men.html

And this is only lvl 1 Patriarchy. At lvl 2 a man may marry as many wives as he wants, and the young single men are just war fodder.

https://www.youtube.com/watch?v=-Im4YAMWK74

Relevant follow-up (videos explore Korean gender politics and hierarchical society).