What problems did the decline in housing prices that began in 2006 cause for the financial system?

Basically, because of the declining prices, there was a rise in defaults among the subprime borrowers, Alt A borrowers, those with adjustable rate mortgages, and those who made small down payments. Because of this, securities that were backed with mortgages started losing value which caused a lot of money losses for those who investment institutions that owned those cases.