Okay, so I actually know the answer. It's that 405N is closed at exit 12 and they're detouring everybody. Of course, the official detour is a madhouse, so there were several other roads equally jammed trying to get to 520 (140th and 148th, at least).

On the upside, driving around trying to find a non-trafficky way home ended up with me finding an Asian grocery store and another Safeway, so... I guess it wasn't all bad. But geez... I'm so not going anywhere else this weekend that involves going anywhere near 520 or 405.

(Just felt like ranting... I'm relatively new here, maybe it's "normal" for this to happen every once in a while.)