"So they can predict the future?"
Not that hard. The mortages that are seen to be doomed are held by households where the mortgage is an excessive percentage of the total income. Let's say 70%, I don't know the exact criteria they use.
If you're paying that much, and the appreciation rate of housing drops, or goes negative, you're likely to lose the house. You don't build equity, so equity loans become one less option. Sure some of those who are paying that much will tough it out, or get a better job, or hit the lotto. Most will go bust.
As a mortgage loan officer, I know there can be issues.
For example, there were lenders that will allow total debt obligations (mortgage plus credit cards, car, other notes etc) to equal 55% of total gross income. (since then many have clamped down to 45% or 50%) That does not include food, utilities, etc.
Um...my take home pay after taxes is about 62% of my income. Now, granted, that brings up another entirely different issue (excessive taxation) but it is a bit insane. So, say you make $3000 a month gross (before taxes.) This lender lets you go to total debt obligations of $1650 for someone whose total take home pay per month is about $1900. They have $250 to pay for food, gas, utilities, and all that?