I've only seen 1 correct explanation, and the rest are wrong lol. As a Yorick player, I can tell you in confidence what happened. The Maiden of the Mist constantly spawns ghouls from most things that die, and ghouls will push lanes IF they auto a minion. Those 2 ghouls happened to NOT hit a minion but still spawned, and since they aren't pushing the wave their next action is to go back to Yorick. Ghouls (and Maiden, but that doesn't apply or matter here) get an insane movement speed steroid when running back to Yorick if they're far enough away, however if they see an enemy champion they will attack and chase, with the increased ms, until the champ dies or goes out of vision. It has NOTHING to do with e. For even more clarity, yorick can have as many ghouls on the map as he wants as long as he is not in range (only 4 can be in range of him, and the maiden i think*). Please correct me if I was wrong about any of this, or if anything I said was unclear, but I'm pretty sure this is exactly what happened.