In the past, women who didn't really want children didn't have a great deal of choice, particularly if they wanted to follow any kind of socially acceptable life. It was considered a failure to many if they didn't get a husband and children.

Therefore there was no particular evolutionary pressure to select for women who actually had a strong biological urge to have children.

But there is now, so after a few generations you end up with mostly those women having children, that genetically passed on desire becomes more prominent, and birth rates increase again. Until overpopulation becomes a new version of the problem people thought it would be previously.