Comment by nostrademons

Could also be unattended arbitrage bots. These are programmed to look at the price on multiple exchanges and if it diverges, buy on the cheap exchange and simultaneously sell on the expensive exchange. Under current market conditions, that means buying (for USD) on Coinbase, Gemini, and other fiat exchanges, and selling (for Tether) on Bitfinex, Binance, and other Tether-based exchanges. When the music stops, they'll be left with zero dollar reserves, zero Bitcoin reserves, and a hoard of worthless Tether.