Hmm...the only reason why the tripod is shaking is due to wind.
1. Tripod's legs not firmly stabilised on the ground (due to uneven ground?)
2. Point of attachment (dunno wat it is called) of cam to tripod not secured...hence shaky
3. Tripod too light for camera
So when wind blows, the whole setup may wobble slightly. If not, keep your hands away from the camera (if you have been holding on to it all the time)
My suggestion: if possible, set the tripod lower to lower centre of gravity and make it more stable. You can also check whether you tripod is stable, if not invest in a good one for great nite photography.
Another alternative if you don't mind. Get a "S" shape hook from the $1 shop to hang your camera bag somewhere at the bottom of the centre shaft of your tripod. This adds weight and stability. I've been doing this for my NDP shots of fireworks from Sheares bridge (if u know how shakey it can be when a bus runs past) and it works miracles.