I can also explain it in a very simple manner.
Wrapping up bad loans and selling them as funds. Everyone up and down the ladder in the banking and credit rating industries got paid fees no matter how bad the loan. Incentive for loan officers, banks, credit companies to go out and gather as many bad loans as possible. They all make lots of money until the house of cards falls.
And who do these Republican financiers blame? The unemployed laborer (most usually a minority) who they targeted for $500,000 mortgages so they could collect their fees, have the credit rating company triple AAA them, and sell them to someone down the line like your grandmother or Iceland.