What does "pegged" even mean unless Tether is always willing to pay 1 USD for one? Then who would sell their tether for $0.99, at a loss?
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
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.