Corporate debt. Fed raised rates and then when it came time to roll over their debt, a lot of major corporations said "Uh, if interest rates keep going up, we are going to go bankrupt because of the higher costs of debt service." The markets had a taper tantrum (the S&P 500 dropped by about 18% in the second half of 2018), and then the Fed reconsidered and started dropping rates again.
Situation is worse now, because it's highly likely that the U.S. Government would go bankrupt if they had to roll over their debt at higher rates, given that the U.S. debt-to-GDP ratio skyrocketed from 105% in 2020 to 130% now.
Sort of. Bankruptcy means you "cannot" pay your debts. The US can get itself into a situation in which for structural reasons it "cannot" pay back its debtors because the political/social costs of the money printing or taxation required are infeasible, e.g. nobody who will do it can get elected. In that case even though it would be technically possible, it wouldn't happen and the US would refuse to pay.
Situation is worse now, because it's highly likely that the U.S. Government would go bankrupt if they had to roll over their debt at higher rates, given that the U.S. debt-to-GDP ratio skyrocketed from 105% in 2020 to 130% now.