The essence of these claims is that mortgages in the securitization pools, or sold as unsecuritized whole loans, did not conform to representations and warranties made about their quality – specifically that the loan applications contained misrepresentations or the underwriting was not in conformance with stated standards.
The potential liability associated with contract claims in securitizations is usually called put back risk because many of the relevant agreements permit the buyer of the mortgages to put them back to the seller at par.