Mordekaiser Ghost + Tower priority Bug
Every champion that has a minion or pet of some kind has the tower aggro to that minion or pet whenever it gets in range or attacks a champion. The code if I recall, is set so the tower has a strict set of guidelines for what it attacks given different circumstances.
In this case, Mordekaiser's ghost was completely ignored by the tower for 10-15 seconds, even after targeting and attacking allied (to the tower) champions. I'm fairly sure that this is a bug given how towers interact with literally every other pet in the game, and repeatable.