Piggyback mortgage financing structures appear to be creeping back into the market, a trend that some observers say could destabilize the industry. Before the financial crisis, many borrowers combined a first-lien mortgage for 80 percent of home value with a second lien of 10 percent or more in order to avoid paying private mortgage insurance. While many first-lien mortgages are still originated with a simultaneous second in recent years, the combined loan-to-value ratio of the two has been capped at 80 percent or less. “From what we’ve seen from lenders who are interested in expanding their customer base, there appears...