Wheels rotating without any order given

Hello everyone, I know there are a lot of topics about this kind of problems but I read through a lot of them without getting an answer to my problem. So basically, I have a robot which is made of a base_link (collada file, but it's similar to a rectangle), 2 caster wheels to the front and 2 rotating wheels to the back. So what happens when I launch the simulation, the robot rotates around the Z axis without any cmd_vel being applied to the wheel or for or whatever. I'm really lost here guys so I request your help. I also add that when I turn on collisions view on Gazebo, it shows that every part of my robot is in collision, even with the self-collide turned on false, and the floor itslef is also in collision (it's the basic empty world). I join you my xacro file so that you may help me, and thanks everyone for reading to this point. I'm using Gazebo 7.0. robot.xacro: robot.gazebo: falsetrue10left_wheel_hingeright_wheel_hinge${wheelWidth+chassis_Width}${wheelRadius}0cmd_velodomodombase_link macros.xacro: