I think you'll find your passing the midline of the graphic while your scaling down. The scale tool works proportionally from the center point of the graphic... From it's current position (100%) to the center point (0%). Once you past the midpoint with your cursor it starts to count in negative numbers which would effectively reverse the object. Just be more careful as you move the cursor, the smaller you make it the closer to that original midpoint you get, the harder it is to keep it from flipping. Zooming in helps.

p.s. The above was based on conjecture of how I believe the Xara code to actually work. As I am not one of thier developers, take that for what it is...