What Caused The Problems?
Here’s a little video that gives one reason:
But it’s only one reason, and maybe not the main one. The greed of certain mortgage brokers could only have had this impact if the greed (and in some cases ignorance) of everyone else involved in those transactions was not also at work.

[...] by mortgages or housing, although they were both catalysts. [Kim - Also see my post about these mortgage issues that are related to the problem.] The real reason was an accounting rule called “Mark to Market” (also known as FASB [...]