Good to hear Cas asking AU Michael about his motivations. I hope they eventually explain what caused him to be so different from OG Michael. Did killing Lucy make him go nuts, or does it go farther back to a different God/MoC story?
I doubt AU Cas will be revisited, but it would be awesome if AU Michael was responsible for him acting deranged and driving while the other AU angels could fly.
My theory is psycho Mike punished Cas for rebelling on behalf of humanity by cutting off his wings as an example to other potentially disobedient angels. That trauma to his true form, the despair of never flying again, and one too many visits to Naomi's chair; all without a human family to love him, and something in his mind just snapped.