It's part of a nationwide(global?) urban renewal. Millenials and their successors are more likely to want to live in urban areas and stay single longer than their older counterparts.
This is causing rents to increase noticeably in nearly every major urban area. Couple that with relatively high pay for tech sector jobs that like to hire these younger folks and you get it even worse in SF, Seattle, etc.
Be careful of overgeneralizing from the subset of the population you interact with in tech. For example, according to this FiveThirtyEight piece from last year, the actual trend is that college-educated millennials are slightly more likely to move to a subset of dense cities. [1] In general, it's unclear how much of an overall urbanization trend there is.
This is causing rents to increase noticeably in nearly every major urban area. Couple that with relatively high pay for tech sector jobs that like to hire these younger folks and you get it even worse in SF, Seattle, etc.