I'm starting to think it is less about foreign investments, like a few other comments have noted, and more about changes at the top end of the middle class.

Along with the large amounts of growth, wages in the upper middle class have really increased a lot. If you look at tech workers, it was pretty common to see average tech employees pulling in $75k-$150k a decade ago; today employees at a similar level and set of skills are pulling $200-350k. As a result, there's a ton of money in the high end available to purchase housing in the area.

Saving for a down payment is a challenge, but less so if it's relatively easy to tuck away an extra $100k a year. This is likely available to a tech earner but not the Seattle hair dresser, unless they have also increased their asking rates for a haircut the last few years.