What you neglect to mention is that most of these appraisals are not fraudulent. I refinanced a year ago to get a lower interest rate. My home appraised for what homes were selling for in my neighborhood. This was easily verifiable, even by me. One year later I sold my home for almost exactly what it appraised for last year.
If the market drops, the lenders are out of luck if they have to foreclose. If they don't have to foreclose, then it really does not affect them since they are still earning a market rate on their investment.
You still haven't told us why a mortgage company would want to foreclose on a performing loan. To do so would be monumentally stupid.
Another far more complex, very innovative (but allegedly criminal) predatory tactic involves predators creating and exploiting conflicts of interest among the various purchasers and servicers of a pool of mortgages, through frivolous foreclosures of performing loans, and legal barratry contrary to fiduciary duty that are extremely profitable for the predators. Citing, Super Future Equities vs. ORIX