Currently the first slice starts at 12 o'clock and extends counter clock-wise. Then the second slice also starts at 12 o'clock but extends clock-wise. The rest of the of the slices then follow the second slice and also extend clock-wise.

This behavior can be observed in the geom_pie docs.

In an ideal world geom_pie receives two additional aesthetics (start and direction) which control at which angle first pie is drawn and in which direction does each slice extend. This would also require fixing the behavior of the first slice extending counter clock-wise while the rest extend clock-wise.