They're very safe assets, they just have a long duration which makes them really risky if you could need them to cover deposits. To make things more straightfo

They're very safe assets, they just have a long duration which makes them really risky if you could need them to cover deposits. To make things more straightforward, let's just compress it to a 1-year time frame vs a 30-day bond. So a $100 MBS at 1.5% would pay you $101.50 if you held it for 1 year. If you have $100 in deposits and a $100 MBS bond, you're "solvent". But what happens if after 30 days, your depositor asks for his $100 back? You either need to sell your MBS or find other money to pay them. If you try to sell the MBS to pay that $100 deposit liability and interest rates are about the same as they were when you bought it, you'd likely get around $100 and things are okay. If however, rates have spiked since then (like they have here), investors can either buy your bond that pays 1.5% or they can buy a new issuance 1-year bond paying 4% or 5%, or perhaps a 30-day bond paying 1.5%. So you need to give them a discount in order to sell your bond -- in this case it might be upwards of 30% or 40%. So if you sell your MBS, you'll only get $60 or $70 for it -- leaving you a huge shortfall that you need to makeup from your other reserves. If you could convince your depositor to leave his money in the bank until that bond matures, you'd be completely fine -- but the timing mismatch and interest rate spike just kills the bank.