No, it was on the touring bike, which I have ridden a hundred times down that hill with absolutely no problem. I think I must have gripped the bars hard as I braked which may have triggered it, possibly combined with things being a bit out of true or something loose somewhere (entirely possible with my bike).
Most people's experience of them seems to be similar - they are terrifying to the point where you start planning how you're going to land when you finally fall off, and yet in the end people ride them out safely. Very confidence sapping though! The advice is to relax your grip (easier said than done), shift your weight out of the saddle, and touch your knee on the crossbar to dampen the oscillation. I've got no idea if I did any of those things in the heat of the moment though