The National Association of Realtors said that its index of pending home sales plunged to 96 from a revised level of 114.3 in October. Analysts had predicted a drop, but nothing like that. “We thought it would drop 2 percent,” said Jennifer Lee of BMO Capital Markets. “When you see 16 percent, the first thing you say is, what the heck happened here?” ... There are plenty of reasons for worry. The Obama administration’s effort to compel lenders and servicers to modify loans has not been a success. Many of these owners will eventually lose their homes to foreclosure. Meanwhile, a quarter of homeowners with mortgages owe more than their houses are worth. If prices start dropping again, some will be induced to walk away, further undermining the market. “I wouldn’t rule out more stimulus, especially in an election year,” said Ivy Zelman, an analyst.