Historically, bond prices rise when stock prices fall. If the stock market is unstable, the funds invested in stocks are concentrated on relatively safe assets such as U.S. government bonds.
But it's not always 'bonds go up when stocks fall'. The saying that stocks and bonds go the other way has been around for a while.
High inflation and recession fears hurt all financial markets in equity bonds.
In general, when stocks fall, bonds rise, so experts advise investing in stocks and bonds in a 50%-50%, 60%-40%, 70%-30% weight, etc.