Also, that $1 valuation was supposed to be backed up by around $4 billion worth of bitcoin. If Terra (the company behind UST) starts selling bitcoin to prop up the valuation, the price of bitcoin will also tank.
And since markets anticipate that this is what will happen, the price of bitcoin is already tanking, which reduces the value of the no-longer-$4B hoard, which means they would need to sell more, and you can see where this is going.