A growing percentage of Santander Consumer USA Holdings Inc.'s subprime auto loans are turning out to be clunkers soon after the cars are driven off the lot.

Some loans made last year are souring at the fastest rate since 2008, with more consumers than usual defaulting within the first few months of borrowing, according to analysts at Moody's Investors Service. Many of those loans were packaged into bonds.

Santander Consumer is one of the largest subprime auto lenders in the market. The rapid failure of some of its loans implies that a growing number of borrowers may be getting loans based on fraudulent application information, a problem the company has had before, and that weaker consumers are increasingly struggling. During last decade's housing crunch, mortgage loans started souring within months of being made, signaling growing problems in the market.