I believe mismanagement and fraud were the reason FTX failed. However, CZ's tweet and announcing that he was planning on dumping $500 million in FTT tokens was the straw that broke the camel's back.

If CZ hadn't written that Tweet, the FTX/Alameda fraud may still be going today.

I wouldn't be surprised if CZ had suspicions about FTX's insolvency for a while. It's interesting that crypto focuses on decentralization and these centralized powers can sway the markets so heavily.