This shaking happens to me after very fast downhills too. No correlation with length of ride, physical condition, hydration etc. I think it's just adrenalin. Some of those downhills are pretty exciting... I have one near my house: relatively short but really steep (you build up speed really fast), traffic light at the bottom of the hill and the road turns very sharply (90 degrees basically) only a short distance after the light. First you're bombing down it wondering if you're going to make the light, and if not how you're going to stop... next you are trying to slow down as much as you need to make the turn but not more than that, cause slowing down more is boring. My legs do shake after that one.