1. Bankers gave mortgages to anybody. Even people they knew couldn't afford it. They didn't care if you would later default three years down the line, which you often did if you had an adjustable interest rate. Joe Schmo at Bank, USA already got his money upfront. Joe Schmo wants to flip as many mortgages as possible, to get as many commissions as possible.
2. Bankers would get swaps (credit default swaps) on those mortgages, which is a basically a warranty, to further protect themselves. Joe Schmo at Bank, USA got your money upfront when you bought the house and he got most of the entire principle back when you defaulted from the Credit Default Swap. Risk free investment.
3. Bankers would also group of all these mortgages together and sell them as one giant bag (securitization) to places like Fannie Mae to further reduce risk/move it from their "books". One of the 3 credit rating agencies (Moody's, S&P, Fitch), which are kinda like Rotten Tomatoes in the financial world, would "rate" the bag of mortgages for prospective investors. Joe Schmo could give out a few questionable mortgages, make his commissions, buy swaps and it all else fails, bury these bad loans with 1000 other good loans and sell the bag of debt to some unsuspecting buyer/investor. Joe Schmo is
4. The problem.....Joe Schmo giving mortgages out like they were skittles caught up to him. The average American could make the first couple years on his mortgage. But when the interest rate (which was often adjustable) spiked up too far, he'd default. Joe Schmo and Bank, USA were now carrying
a ton of subprime debt (worthless loans). The ratings that the credit agencies gave those loans were manipulative, which fooled alot of firms/investors into unknowingly buying toxic debt. Investors had now caught on and banks couldn't move the toxic weight as easily anymore. Now Joe Schmo realizes he is fukked and goes crying to Capitol Hill that he needs a bailout or else the financial sector is going to collapse. That's real rich coming from you brah
5. The next problem. Almost like a bank run, all of these holders of the toxic debt swarmed places like AIG to exercise their credit default swaps. Which again, is a basically a warranty that offers back some/most of your money in the event of a default. Though AIG gave out all those credit default swaps, when the time came to pay out, they didn't have enough money

Enter another government bailout.