This is what I found via TurboTax:
How the IRS classifies cancelled debt
You might consider it unfair that a debt you successfully cancel or negotiate away comes back to haunt you as taxable income. However, the IRS classifies cancelled debt as income because you received a payment you didn't return.
When you first borrow money, you don't have to pay tax on the money you receive because you are bound by a contract to pay it back. Once that contract no longer exists, the money is yours to do with as you please. Since you essentially received income for free, the cancellation of your obligation to pay it back makes it taxable.
Form 1099-C
According to the IRS, nearly any debt you owe that is canceled, forgiven or discharged becomes taxable income to you. You'll receive a Form 1099-C, "Cancellation of Debt," from the lender that forgave the debt. Common examples of when you might receive a Form 1099-C include repossession, foreclosure, return of property to a lender, abandonment of property, or the modification of a loan on your principal residence.