Those der spiegel articles and leaked emails. Once they were out, Uefa reacted by opening their investigation. As soon as they did, the PL opened their own investigation as well.
I honestly thought once CAS closed that episode, the PL would follow suit and use that. It would have been quite easy for them to close it out and claim, consistent with the CAS findings, they conclude the same.
But they didn't, they took another 2.5 years, investigating books right back to 2009 before even uefa's ffp came about. Which is why I don't buy this they are only doung it to appease pressure. I'm sure there IS pressure, and they are trying to appease it. But if that was all, they could have fallen in line with the CAS findings and closed it and still been able to say they tried.
Re doing it in reverse as you mention, I don't think pressure alone is enough. There has to be a big enough public thing to make them have to be seen to act. I.e You can't just request they look into a club for a laugh. Unfortunately, ours was thatnder spiegel leaks which clained to have actual evidence of wrongdoing and put it out in public.