The classification seemed relatively simple. The Ukrainians were already outcasts. The Communist Army had imposed their will on the Ukrainians when they overthrew the Ukrainian ruler. The Ukrainians themselves had rich culture and thus a large and deep sense of national pride. The deposing of their leader made them bitter to the idea of communist rule. Their independence had been attacked. Due to this, Stalin felt their national pride undermined his power and the idea of communism as a whole. These traits made the Ukrainians an easy group to target.
A map of Ukraine under Soviet rule