I've been wondering about some of the specifics on this. One thing I've heard is that the UK pension funds for DB (defined benefit) plans had an issue as rates fell. Typically, pension funds will use long term bonds for a good portion of their expected payouts. As rates fell, it became impossible to keep the same bon return -> income flow, so the pension funds would have had to:
a) Require higher contributions for a given expected benefit, or
b) Reduce the benefits.
Neither of these would have been acceptable, so they needed another approach.
One way would be to buy "riskier" assets than known maturity bonds. For example stocks.
Another approach is to leverage up on their (now lower coupon) long bonds by borrowing short and using that to buy EVEN MORE long bonds. With short term rates near/at zero, this was an effective strategy.
All is fine and dandy with such an approach UNTIL short term rates rise, and especially bad if short term rates exceed long term rates (i.e. an inverted yield curve). Now they have a negative spread (oops) between the borrow short and buy long strategy.
LEVERAGE KILLS.
The BOE realized that the pension funds had a big big problem, so what to do? Print money, i.e. QE. So much for tamping down inflation and QT.