The demand for Tethers is high because the demand to be able to trade is high.
> Do you think more and more people are selling BTC for USDT for a "short term"?
More and more people have been getting involved in trading as the price of Bitcoin and other cryptos was spiking. Now those people are in the market, but Bitcoin has been dropping. It makes sense that a lot of them would want to hold a USD proxy under these circumstances.
Of course, that's the optimistic scenario. It's also possible that There's are being used fraudulently to pump BTC. Without proper transparency, it's impossible to be sure.