Consumer debt is the obligation to pay for the credit that the customers have used. But, how can tapping out more credits make business more vulnerable?
In the long run, consumers cannot spend more than they earn. In the short run they can spend more, increasing consumer debt, or they can spend less, decreasing consumer debt.
There is a limit to how much consumers are allowed to borrow. The limit is determined by consumer income and the interest rate.
If consumer debt increases faster than consumer income then eventually consumer debt will peak; bank and credit companies will stop lending more money causing a halt to the spending spree.
Note that consumer credit tends to increase market fluctuations. When times are good more people have jobs and consumers with jobs are allowed increased credit. When times are bad you see the opposite.