What is the definition of a default here? Legally, a default is when a firm cannot pay a due payment which it is legally obligated to pay. Tether LTD. has no obligation to redeem tethers, and owners of Tether have no claim on Tether Ltd as they say in their legal page:
"Purchase and Redemption of Tethers: The Site is an environment for the purchase and redemption of Tethers. Once you have Tethers, you can trade them, keep them, or use them to pay persons that will accept your Tethers. However, Tethers are not money and are not monetary instruments. They are also not stored value or currency.
There is no contractual right or other right or legal claim against us to redeem or exchange your Tethers for money. We do not guarantee any right of redemption or exchange of Tethers by us for money. There is no guarantee against losses when you buy, trade, sell, or redeem Tethers."
So by the legal definition of default, it doesn't look like it's possible for a tether token to default, since it's not a claim on anything. Tethers are not I.O.Us.