why the box turn over when set velocity?

edit

thanks， but how to change the coefficient of friction? In the gui, I did not find how to change it.

You can set it in GUI:

right click on the model -> Edit Model  -> right click on the link -> Open Link Inspector -> Collision tab ->  drop down the collision -> Srurface/Friction/Mu edit value

Or in SDF code:

<model>
...
   <link>
    ...
      <collision>
      ... 
         <surface>
             <friction>
                 <ode>
                     <mu>1.0</mu>
                 </ode>
             </friction>
        </surface>
...