The simplest possible explanation is basic statistics so the top 20% of those bonds supposed to fit AAA criteria. No conspiracy. No "unidentified group of people". No corrupt reasons, just legitimate profit seeking and extraction.

It just naturally happens that that legitimate profit seeking and extractions benefits from the actions like "the indices are supposed to track the market, they can't do that if they exclude these IPOs", and i described the natural simple arithmetics how it happens. No conspiracy. Just arithmetics. You can verify it.