What is co-holding and why it's puzzling

Do you have both a credit-card balance and a savings account? Would you use your savings to pay down your credit-card balance if it would save you money on fees? Chicago Booth's Abigail Sussman talks about her research that suggests that about one in five people holds a credit-card balance and a savings account, which effectively means they are paying to borrow their own money. What would it take to change their behavior?