Originally Posted by
FBinNY
Another possibility it that there was always a slight tendency to wobble that you've always controlled, but with fatigue you're ability to manage it got worse.
Sounds like your mechanic has hit all the usual mechanical suspects. I'm guessing it's the fatigue factor, multiplied by how tense you got after that first incident. You likely tensed up, probably because you were slightly fatigued. Next downhill, you tense sooner, and set up that bike/rider resonance sooner, which makes you even more tense on the next downhill.
You can damp that out with the knee on the top tube trick, but the real solution is to loosen up. Sometimes that's not possible (me, downhill at 35 mph, 15 mph headwind, 40 degrees), but if you can relax that death grip on the bars, it'll let up.