I would say that was a symptom rather than a cause - once people stopped caring about the quality of mortgages (because they would be sold off and packaged up and sold on) it was pretty much inevitable that this would happen.
They can’t be sold off if they never passed the underwriting tests in the first place. Intentionally not verifying incomes and/or fraudulently staying higher incomes is what allowed for plausible deniability to rate them higher than they should have been.
