Tether will collapse when withdrawals of USDT exceed the amount of assets they have. So all that's needed is another Luna-style shock to the system that trigger enough whales to withdraw.
Why? They may have acted reckless up to now with the commercial paper etc. but unless they have taken massive losses and somehow covered them up, they can just convert to risk-free assets and take the yields.
Why? Because if Tether has $10 billion in actual assets, hands out $20B worth of USDT, and whales redeem $10B, the remaining $10B USDT are worth nothing.
And, yes, it is virtually certain that they have taken massive losses (seen the state of the crypto markets and ecosystem lately?). The big unknown, as ever, remains how much actual assets they have and how liquid they are.
Tether is not backed by crypto so why would crypto market losses affect it? They publicly state what their assets are and how much they're worth. People just think they're lying for whatever reason.