Americans are suddenly defaulting on their credit cards

The American economy has looked pretty robust of late — unemployment just hit a 16-year low and stocks recently reached an all-time high.
This makes it all the more curious that Americans have suddenly stopped paying off their credit card bills at a rapid rate. 
In the last two fiscal quarters, banks reported a steep rise in credit card charge-offs — debt that companies can't collect from their customers — according to a report from Moody's. 
This chart from