Jeff, I have to say I'm not surprised.
Filling with sand is not the best idea. I'll try to explain the physics so you (and hopefully others) may understand:
Vibration amplitude (the size of the vibration) is proportional to the exciting force (a bump, or periodic force) and inversely proportional to the frequency of vibration.
The frequency of vibration increases with stiffness and decreases with mass. So when you add mass (by filling with sand) you decrease the natural frequency of vibration or the tripod/pier. This means that any given size exciting force ( a bump, etc) will result in greater amplitude of vibration.
Get rid of the sand!
Al.