All those years of rising U.S. auto sales are starting to work against carmakers. A glut of used vehicles has started to depress prices. That trend will intensify as Americans will return 3.36 million leased cars and trucks this year, another jump after a 33 percent surge in 2016, according to J.D. Power. The fallout has already begun, with Ford Motor Co. shaving $300 million from its financial-services arm’s profit forecast for this year.