Because the 10 year gilt rate is up at 4.5%.  If you buy your own bonds, it lowers the total coupon, and therefore is deemed lower risk

It's not the absolute level of the gilt rate, it's the speed at which it moved

https://bondvigilantes.com/blog/2022/09/collateral-calls/

FT have just done an article on this as well

https://www.ft.com/content/038b30c3-f550-4cc0-93ed-9154021d6ee2

Thanks Vertigo, that's actually a very interesting and useful set of links. 

[Checks is actually on RoF]

Nationalise the BoE and do it as soon as possible

Same reason the mortgage market has seized up.  Banks can't manage the admin of risk management while the swaps market is so volatile.