6dof produces rotation in quiescent fluid
 

To familiarize with 6dof, I am trying a very simple case involving a rod hinged at one end. The case is 2D transient, pressure-based, zero gravity. The model is simple, just four stationary walls that forms a chamber, enclosing a bulk of air. The rod is in the middle of the chamber. There is no flow.
The rotation axis and center of gravity are both (0,0). The moment of inertia of the rod is 1.
When dynamic mesh is turned off, everything is fine, there is no flow, and there is zero pressure everywhere.
However, once I switch dynamic mesh on, and activate one-dof rotation for the rod, remeshing for the surrounding mesh, something strange happens. Somehow, the solver decides that the rod should rotate, and produces very high pressure (~1e5-1e6) above the rod, and low pressure below the rod. Checking the dynamic mesh zones setting, I can see that there is a rigid body angular velocity of ~5, i.e., the rod is rotating.
My questing is, why is the rod rotating even in quiescent fluid? What might the problem be? I re-emphasize that there is no fluid flow, no gravity, no buoyancy, no spring load, zero gravitational acceleration, nothing that can drive the rotation. Just a rod in a chamber filled with still air.