It's borne of a bit of a misunderstanding of what was going on. Some pension funds were having to sell some liquid assets, including gilts, to raise cash for margin calls on interest rate swaps that had turned against them cos of rate projections leaping up. It was more a fear that people selling gilts would force the yield up higher, becoming self-fulfilling, so the BoE intervened to buy a load and try to put a lid on yields.
@11101 will be able to explain far better, but you can't have a run on pension funds per se, so the Northern Rock example isn't great, but it sounds like a serious funding crunch was happening and the BoE had to act. These pension funds are multi-billion schemes, so even small percentages for them are colossal amounts of money.