Yes, yellowjacket wasps! My daughter identified them as they were attacking. I see yellowjackets often hanging around water fountains, etc., and they were never aggressive. Anne guesses they thought we were attacking their nest. Maybe the dog or one of us stepped on it.
My wife called the city of Los Altos and they were so nice about it. They asked exactly where it was and then they dispatched a crew. They called today to say they found it and took care of it, whatever that means.
Today, standing under a flowering tree and talking to Anne, we suddenly both noticed a loud buzzing like we heard during the attack and looked up to see hundreds of bees buzzing around our tree. We both gingerly left the area. But it turns out we have a zillion bees in our yard all the time, I just was never sensitive to the background buzzing noise before this wasp incident.