But like the CEL (Celsius) a lot of the "market cap" of the coin is held in companies that are essentially insolvent. Like CEL, they use their big balance sheet and the relatively thin amount of trading to prop up the value of USDT when necessary.
That's just an idea. I am not someone who would know. One thing I have learned in life is that people can know there is a problem in a market but the chickens don't come to roost until the last person is convinced they can't make money. Mortgage backed securities were like that. For a couple years people thought it was a problem, then probably most financial people thought it was a problem, and then a long time later it crashed. It really was waiting for the last person to think they couldn't make any money by trading them.