At least here in Norway this trend probably started even earlier, but we have a significantly larger proportion of dual-income university-educated couples. (This trend is supported by our one-year parents leave with pay, where the parents have to share this time, and by public kindergartens when the children are a little older.)
I suspect that a strong driver for this big city concentration is the fact that most couples meet sometime during their university studies, and when this switched from being men getting their MSc's meeting the girls from the nursing schools, to being men & women at the same university, they would have really strong incentives to try to settle in a city with a big enough employer base that both would have multiple job alternatives.
I.e. my wife & I have lived in Oslo for almost 30 years now, we have always had lots of employment options, while my youngest brother and his wife live in a far smaller town:
In their area it has significantly harder to locate alternate (and interesting) employment when bad times hit the company one of them worked at.