Apparently, a number of lenders bought mortgage insurance on particular mortgages from private mortgage insurance companies. To clarify, the lenders did not require borrowers to purchase the mortgage insurance, but rather bought mortgage insurance (and paid the cost) on their own.
Under the terms of the policies, the lenders get a pay-off from the PMI companies is they foreclose on a property, but not if they modify a loan or allow for a short sale. Consequently, lenders are better off foreclosing than modifying, even if the foreclosure produces lower proceeds than a modification.
This is yet another perverse incentive that is contrary to the policy aim of stabilizing the housing market. I have no idea how widespread this is, but if it is common, it is yet another problem.