Why turn off Image Stabilization when using a tripod?
Turn off stabilization when on a tripod to eliminate the possibility of camera shake. The image stabilization (or VR) introduces vibrations to detect camera movement and this may create a "feedback loop" if your camera is already steady on a tripod. (Check this article for further information)
The only exception is when your camera has a tripod detection feature. Please refer to your Camera Manual to see if you have it.