That rules out some possibilities. At this point it sounds similar (not identical) to recent shutdowns that seem to be algorithm driven, some of which have been reversed when people looked at the accounts. I would just wait for the letter.
letter came back as ""After careful consideration, we closed your credit card account because our affiliate is ending its banking relationship with you.""
seems like some kind of rare shutdown imo.
TL;DR CIP shut down. Personal cards are still okay.