When the housing bubble burst and house prices started declining, borrowers began to default, the lower tranches were hit with losses, and higher tranches became more risky and declined in value.

• 房地产泡沫破裂的时候、当房价下降的时候，借款人开始违约，低层的“块块”被市场击中，发生亏损，高层的“块块”风险增大，价值下降。