If people think at all rather than just doing what everyone else does. People invest in their future. Peasants don't own land, can't own land, have zero access to financial wealth or education, so they try to breed because adult children are the only type of family wealth they can produce.

Baby boom is those people with that mindset with some sudden prosperity.

Doesn't last as soon as they see the successful people invest in land, financial assets, material goods, and children's education. Base culture matters, you saw Confucius based cultures turn on a dime once they had two to rub together.