UK had an unprecedented intervention by their central bank in their treasuries market. Apparently their pension funds were highly leveraged and long these, due to some accounting quirk that tried to avoid accounting “losses” in favor of real ones. When rates rose, and their long bond positions fell, they nearly all were going to get margin calls and have to blow out of their bonds and this could trigger cascading selling. This was a good explanation-
https://www.bloomberg.com/opinion/articles/2022-09-29/uk-pensions-got-margin-calls
Other coverage
https://www.wsj.com/articles/the-return-of-inflation-makes-deficits-more-dangerous-11664366538
at one point on Wednesday morning there were no buyers of long-dated UK gilts. “It was not quite a Lehman moment. But it got close.” …
“If there was no intervention today, gilt yields could have gone up to 7-8 per cent from 4.5 per cent this morning and in that situation around 90 per cent of UK pension funds would have run out of collateral,” said Kerrin Rosenberg, Cardano Investment chief executive. “They would have been wiped out.”
More broadly the issue is that rate hikes are making the financial system unstable in cases where the country already has large budget deficits / debt.