So, after examining the battlefield, I think here is what most likely happened:

Your opponent has cast a noncreature spell. This triggered Displacer Kitten to exile and return a permanent they control. They chose Eaten by Piranhas that was probably attached to some creature of yours. Since the aura entered in a way other than as a spell from stack, there was no target chosen for it. So the game let them choose a legal object to enchant. Just legal to enchant, meaning possible to be legally attached to when on the field. No targeting involved, since auras only target as spells. This gets around hexproof and shroud (protection would help, though).

303.4f If an Aura is entering the battlefield under a player’s control by any means other than by
resolving as an Aura spell, and the effect putting it onto the battlefield doesn’t specify the object
or player the Aura will enchant, that player chooses what it will enchant as the Aura enters the
battlefield. The player must choose a legal object or player according to the Aura’s enchant
ability and any other applicable effects.