This is a headline from Reuters, citing a source familiar.
The pension stuff has been doing the rounds all day so there's enough smoke here to imagine some fire. What it sounds like isn't direct pension bond holdings but a pension (or pensions) with investments in a fund that would have blown up today without a BOE bailout.
Whatever it was, it was bad enough to send the BOE into action. Of course, this is also the kind of thing you'd leak if you were looking for cover -- no one wants to see pensions hurt.