Why does Darius E (Pull) focuses Minions over champions?

When I Pull a champion near a minion Darius focuses a Minion instead of the champion I pulled. There was a Minion behind me as I pulled the enemy champion towards and when I pressed W he attacked the minion behind me instead of the champion. This has occurred numerous times even with normal auto attacks why does he prioritize minions over champions after a pull if any are near him?