More USDT got issued because there was opportunity to make 0.1-0.2% on each new USDT since demand for it was that high (i.e. people were paying real USD to buy USDT at a premium, imagine that).
USDT (and USDC) demand spikes during crypto crashes because people actually cash out to these instruments because they are so widely used and trusted by actual market players. People aren't cashing out of crypto ecosystem during most crashes, money actually stays in crypto networks.