Folks who want their fiat.
It’s been sitting at 99.9 for several days now.
Depending on how many people want to sell, how quickly they want to, and how many market makers are willing/able to arb it back up to $1.00, you might just take $0.99 instead.
As for why would someone buy or sell for other than $1.00? Plenty of reasons, if a market opportunity to make 10% in a crypto investment pops up, damn right people will liquidate tether at a slight loss