There is definite movement there, that is not unexpected. There are three main pieces to the tail of the car: two wheel fairings and one wing. The fairings are bolted to the wings and thier main function is to cover the wheels. Rotational movement or lateral (camber) needs to happen independent of the leading edge of the wing.
This will change as speed increases and the axle is loaded with the prescribed down force. When the loading is applied, the camber will become stablized/fixed, but the fairing will still have the ability to rotate about the axis of the wheel.
We are more interested in making sure the leading edge / angle of attack of the wing is controled and carefully maintained. The wheel fairings need to remain perpendicular to the wing. If they move from perpendicular, they need to do so evenly and controlled. This is an area that we can only monitor through higher speed passes since it is all driven by aerodynamic forces.
Low speed passes will only give us so much data. When we get some loading on the wings, then we'll know more about what is going on there. This is all part of the testing program on our way to our target velocity.