It's been a cold week, at least by Seattle standards. And several days of weather in the 20s is all it takes for the lakes to begin to respond. It's been a while since I've seen Green Lake with this much ice on it (although I still recall the whole lake safely frozen over in 1983 and/or 1984, when I first arrived in this neighborhood).
The ice was pretty thick here at the south end so it probably wasn't too crazy for folks to be walking on it. At least those who had some idea of what to they were doing. But some of the people checking it out were treating it the way that some people tromp around in the snow for the first time. I'm not sure they had any clue of the consequences of getting it wrong!