Well, he lost his memory and started manifesting this stuff after they found him unconscious in the library, so that was likely the key event. Assuming Gluttony was already in the tower, she could have done just about anything to him while he was out cold. Alternatively, this could have been set up as far back as the previous season, and when Subaru got knocked out by reading a book, it provided a chink in his psyche that allowed the setup to bear fruit, so to speak.

The other possibility is that it had something to do with everyone’s favourite chopstick samurai.

With this being two consecutive cours, it might be a while before we get the answer.