We've found, from what has happened in our neighborhood, is that people who have lost their jobs and don't have income beyond welfare, can't keep up their mortgage payments. Homes here are expensive - median 3-bed, 2-bath 15 years old is around $250K.
Once the mortgagee becomes delinquent beyond a certain point, the lender forecloses and the owners are evicted. Why the mortgage lenders don't actively pursue re-selling them in our hot real-estate market is a puzzle.
In most cases here, there's such a demand for houses, that legitimate sales, by owner through a realtor, don't sit empty for more than a few months. The exceptions are repossessed homes and those (and there are some in our neighborhood) which have been invaded by drugger squatters who turn the houses into meth labs. I don't know how the mortgage companies deal with the remediation costs. It would probably be cheaper to burn the places down!
Fortunately, those cases are in a very small minority (two in a neighborhood of a couple of thousand homes), but it's very distressing for the immediate neighbors.