Who do you think are selling?
Institutions got in between 5-10k and when it gets too risky then they are the first to sell. That's just how it is.The price would fall because several exchanges and Tether bought Bitcoin for free. People would get scared and sell - liquidity will take a heavy hit (Tether provides a really high percentage of liquidity). High buy orders would get bought up/cancelled fast, people who hold Tether cannot exchange it to anything anymore.

Tether is also having a RICO charge and a missing funds accusation thrown at them.
https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.courtlistener.com/recap/gov.uscourts.nysd.524076/gov.uscourts.nysd.524076.154.0.pdf&ved=2ahUKEwjA84Ty75PuAhUG_CoKHZG-Ay0QFjANegQIBxAB&usg=AOvVaw0v_A9CdhZrtYaDNF3xMeNO

Tether has to provide documentation to NYAG on 15.01.2021

Tether is really shady and that's a fact.
You can listen the podcast Paolo Ardoino was in and basically an hour after the podcast Bitcoin started dropping. Podcast https://www.youtube.com/watch?v=b_Uq5QCHcU0 .

They say they are backed by an "asset mix" including BTC. Answers they give about more serious questions don't really answer anything.
They have never had any audits by any trustable company. The only thing that is saying they have some money is a piece of paper from Deltec Bank (https://static.cryptoglobe.com/filer_public_thumbnails/filer_public/bf/a0/bfa004d4-52af-451b-97d8-25a4de6f1ebb/deltec_letter.png__800x806_q85_crop_subsampling-2_upscale.png . Deltec has been accused of money laundering https://www.dfid.org/news/deltec-bank-rumoured-to-be-connected-with-odebrecht-money-laundering
They will give 2.5mil (yes, 2.5 million) papers to investigators, seems like a delay tactics (same thing in the Wolf of Wall Street where FBI was going through the papers in the room, when AC was pulled down to low.).
And the bank statement that there hasn't been any money coming in but Tether has printed billions https://i.4cdn.org/biz/1610310413780.png .