Here's a study that performs analysis showing that tether prints the coins and someone at bitfinex uses them to buy BTC at optimal points to support it's price.
Remember this is newly printed tether which means net deposits of new actual usd, if you believe them - and of course I do not.
In the time since it was published, when Tether had issued about $4.5 billion total, there have been over $14 billion additional Tether, a 4X expansion of the total supply, or 7.5X what was described as "large swaths of Tether" in this article.