For months, the steady drip of news about troubles in the subprime mortgage market looked no worse than one would expect: merely a comeuppance for lenders, borrowers and investors who should have known that high-interest loans to people with poor credit were risky. During the same period, many economists started breathing again after concluding that the superheated home market of recent years had not become the bursting bubble many had feared. While home prices are leveling off, there has been no deep, widespread decline. But now some experts wonder whether those sighs of relief came too soon, especially in light of the troubles recently experienced by one of the largest subprime players, HSBC Holdings. Some suggest that the growing number of borrower defaults in the “aggressive lending” market, which includes various types of risky mortgages besides subprime loans, could shock the broader housing market and economy after all.…