wt was the cause of the sub prime mortgage crisis in the US

You are right that the failure of the mortgage borrowers to repay the debt caused the sub prime mortgage crisis.

Sub prime mortgage is a kind of mortgage with poor quality and high risk. The original purpose of sub prime mortgage is to help the poor to own their house. So long as the demand of house keeps growing, the developers will keep increasing supply. It will create more job opportunities to construction workers and lower the unemployment rate. When these people have money, they will spend and bring to the gowth of the national economy.

However, now it turns out the opposite happened. The borrowers were unable to pay the mortgage. The banks take over the houses. Demand of houses then fall, and the developers are reluctant to build houses. Less construction workers are required and unemployment rate increased. People have limited money to spend and lead to the national downturn in economy.

Luckily, sub prime mortgage is not allowed in HK. Otherwise, I can't imagine......