Why Are Small-Bank Credit Card Delinquencies Up?

Our friends at the St. Louis Fed present us with a mystery this week:
In the third quarter of 2017, small banks saw a huge surge in credit card delinquency rates. Nor was this just a weird spike: delinquencies have stayed high ever since. Meanwhile, big banks have seen no change at all. What’s going on?
The St. Louis Fed folks don’t know. I don’t know either. But something sure seems to have happened in the summer of 2017. Anyone have any guesses?