The main driver is ETFs that are constantly buying. Tether could be contributing if they really hold as much bitcoin as they claim, because that would mean they always have to get more BTC to print new USDT, but who knows what they're really doing. We'd need regular audits for that.
I don't trust Tether.
Tether always puts new USDT in circulation. Other stablecoin issuers are more conservative in this regard. I wonder if Tether would be able to prove it really holds the USD it claims to have in its reserves? If it keep increasing the supply, it must provide regular audits like you've said. Something fishy is going on here.
I sure hope USDT doesn't lose its peg to the USD. Else, crypto market prices would decline at a very fast pace. The damage will be so severe that it will take quite some time for major crypto assets to go back to their original prices. It will be a great buying opportunity for new investors, but a huge loss for existing crypto holders. The market is widely unpredictable, so we can only hope for the best.