No one knows how tether really backs up their USD. If they use actual USD they're at the mercy of legacy banks that can shut them down and if they use BTC or others assets to back up tether then they are at the mercy of the markets and the prices of their assets. Tethering ain't ezpz.. Now big boy goldman sachs has entered the stable Ecoin (Evil coin) game giving tether some competition. The chart shows that the tether drama arose shortly after the introduction of USDC. Some bigger players probably sold their Tether for USDC and bitfinex et al couldn't keep the peg without selling too much of their assets such as btc.