Now that I think about it the sabre flaps are located on the inboard section of the swept wings so they are quite far forward in relation to the centre of pressure of the wings. The inner most section the flaps may even be in front of the centre of pressure.
If the centre of gravity is close to the centre of pressure, and some portion of the flaps is location in front of both, it would make sense for the pitch up with flaps down. I think...