The percentage of borrowers underwater on their mortgage declined during the second quarter, but that welcome change of pace could come to an abrupt end as home values are again beginning to fall in markets across the country. Zillow reported Monday that as of the end of June, 21.5% of single-family homeowners with a mortgage owed more than on the loan than their home is worth. That's down from 23.3 percent in the first quarter this year, and 23 percent during the second quarter of 2009.