When you attack or attempt to attack an enemy Illusion, it'll draw aggro from lane creeps.

Repro Steps:
1) Create any hero for yourself and create a Phantom Lancer (I only tested it out with this guy).
2) Go to any lane where there are lane creeps.
3) Have the Phantom Lancer pick up an illusion rune or use any of his first 3 skills.
4) Have your hero attack an illusion (make sure they were attacking something else beforehand).

The lane creeps will get aggro'd as though you tried to attack a real hero.

Attacking an illusion isn't supposed aggro creeps.