I mean, people who get married typically plan to have children next. At a minimum they tend to stop participating in many urban activities, night life, etc.
Having children in an urban area is more expensive and the schools tend to be worse.
I don't think this is a uniquely Seattle phenomenon. It's pretty common to emigrate from the city center to the suburbs when planning to start a family or otherwise "settle down".
The finance people from my cohort from grad school moved to Manhattan and thereabouts to a large degree. 10 years later? I know one couple who still live there I think.
Having children in an urban area is more expensive and the schools tend to be worse.
I don't think this is a uniquely Seattle phenomenon. It's pretty common to emigrate from the city center to the suburbs when planning to start a family or otherwise "settle down".
In most cases it's just simple economics.