They were paying attention. Tether is not real USD and can have a supply and demand issues when BTC takes wild swings on an exchange. It's goal is to be EXACTLY $1 but that never happens. https://coinmarketcap.com/currencies/tether/ Sellers are sometimes forced to trade with a 10-15% hit but they just swap into another coin that also has a 10-15% discrepancy. They balance out as the discrepancy can be across all the coins but then slowly corrects with time. When BTC quickly jumped to almost $20K people tried to convert to USDT but there was a massive shortage on exchanges. They took massive hits and Tether shut down for many hours so you couldn't even cash out.

Holding Tether is very risky. You have to realize you're taking a calculated risk when converting to it. Most likely 100's millions of fraudulent Tethers are being used for margin lending, BTC price manipulation and fake transactions. Someday it will be headline news.