...the proximate cause of the Housing crisis were 1) Ultra-low rates; and 2) Abdication of traditional lending standards, thanks to 3) originators ability to resell mortgages for securitization purposes, and hence, 4) not have to worry about loan defaults.
The credit crisis was caused by 1) the above securitized mortgage paper, that was 2) rated triple AAA by Moody's and Standard & Poors, which then 3) Which was then "insured" by credit default swaps (CDS) -- the unreserved for, shadow insurance products 4) whose exemption was made possible by the Commodities Futures Modernization Act. That legislation exempted these derivatives from any supervision or regulation. The lack of reserve requirements is why there is now $62 trillion in CDS, many of which will never pay their counter parties the promised insurance.
A Very Short Summary
One very succinct perspective on the causes of the current turmoil in the financial markets (from Barry Ritholtz at The Big Picture: