object rotate by itself in gazebo，i don't know why.

I'm using gazebo7 in ubuntu16.04. I launch a bannana model in gazebo,everything is ok but the bannana will rotate slowly by itself. urdf below: Gazebo/Yellow launch file below: Here is the [workspace link](https://github.com/TouchDeeper/objectPoseEstimation.git),and there is a bannana.gif in the home directory to describe the question. Can you give me some advice? Thanks a lot!