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.