Mt.Gox started having issues withdrawing funds to "real" fiat currencies, people got scared, wanted to pull out their bitcoin, suddenly their account says they have a balance, but there's not bitcoin backing up the balance. (In Mt.Gox case, this was because a significant amount of bitcoins were stolen from their treasury.

Similar thing could happen with people redeeming USDT for USD if they don't have enough to cover the already issued USDT.