Does Credit Suisse qualify as a "big bank" anymore? The company could be argued to be circling the drain for many months, but nonetheless holds hundreds of billions in assets. If it failed, it would be a big deal, and financial markets asked that question in a very serious voice today.
All other things being equal, when financial markets are concerned about a global systemic banking crisis, it tends to put downward pressure on stock prices and bond yields. Bond yield is another term for interest rates.
The bonds that underlie the mortgage market don't react quite as swiftly as US Treasuries to these sorts of episodes, but the reaction was plenty big nonetheless. The average lender moved back down to levels seen on Monday. At the time, those were the lowest rates in more than a month.
Whether or not rates remain in this territory (or lower) depends on the emergence of additional banking drama as well as the path of inflation and the economy. The only safe bet in the short term is for volatility to be elevated, for better or worse.