The first CDOs were sound. They were also profitable, which led to others copying the CDO model on a massive scale. Then the competition for loans to build them on got very fierce. This caused the financial sector to burn through all the good borrowers very quickly. Then they began bottom feeding, having deluded themselves that their derivatives had eliminated risk. When you get greedy and want to keep making big bucks, you tell yourself that making loans to bad risks is okay.
As I said, the first CDOs were sound, and so the inventors went to the regulators and "proved" they had virtually eliminated risk, and thus they were allowed to exceed the leverage to which regulations had previously constrained them.
But just to be extra special safe, they bought "insurance" against their CDOs. And they bought that insurance in the form of credit default swaps (CDS) from AIG.
Since the risk really was such that the Universe would blow up before these first CDOs would default, AIG looked at those CDS premiums as free money. They were more than happy to take this new revenue stream from the broker-dealers. And because they felt there was no chance the CDOs would ever blow up, they did not set aside any money for that eventuality.
And that was all good, at first.
But once greed set in, and the good borrowers were all used up, and competition became fierce to find borrowers to take investor cash in the form of loans (mortgages), the quality of those loans began plummeting. Thus the CDOs built on them began to be stuffed with toxicity.
AIG was not paying attention. They kept insuring all these CDOs in the mistaken belief they were as sound as the originals. And so they continued to fail to set aside any capital cushions against all these CDS they were selling.
Finally, in late 2005, two subordinates at AIG FP were able to get their boss's eyes open to what was really going on. Once he realized the CDOs they were insuring were chock full of toxic loans, AIG announced they were no longer going to insure any more CDOs going forward. AIG believed they had caught on before it was too late.
They were wrong. Fatally wrong.
So did this finally get Wall Street to stop building these CDOs?
Nope.
By this point, they had drunk their own Kool-Aid and began selling CDS to each other! Those who had caught on began building fraudulent CDS and synthetic CDOs to pass the risk to unwitting investors. These are the assholes who should be hung from lampposts.
Instead, they have been fined lunch money. Their fines have been smaller than the profits they made from their fraud.