Here's a good comment covering it[0]. I got my summary slightly wrong; buildings aren't re-financed, but rather the owner has the ability to delay payment on any unleased units. Leasing a unit at a lower rate suddenly modifies the value of the building, which means signing a lower lease can put the loan underwater. The same basic motivation structure applies though, it is far better for the owner to leave a unit empty and pray that someone can rent it at market rates later than it is to lower rates and get a tenant now.
