Why do Azir's soldiers cause minion aggro?

Azir's W is supposed to be his main tool for zoning and harass yet in early levels, when you land those hits you end up getting minion aggro. What should be a won trade actually ends up being a loss because of all the caster minions shooting at you. This does not make sense when every other AP mage can cast their spells without attracting minion aggro.