Model moves and rotates when inserted

Hi all

I made a differential drive robot with meshes for bot collision and visual elements. When it is inserted, the robot keep moving in some direction and rotates in its own axis. Both wheels are also rotating even if velocity is not specified. Sometimes it moves forward and backward.Here are some [screenshots](https://drive.google.com/folderview?id=0B5du_0ymFMtvMC1MNG1EcC04TTg&usp=sharing) and the [model directory](https://drive.google.com/folderview?id=0B5du_0ymFMtvTDA3ZHl2X1N2Qjg&usp=sharing) of the case. Properties of each link can be found [here](https://drive.google.com/folderview?id=0B5du_0ymFMtvWjlwS2lOSmtBczg&usp=sharing). How these errors can be resolved?

Thanks in advance