US bond rates hit an ATH in 1981 around 15%. Unsurprisingly, the stock market hit a low at the same time (you should expect an inverse relationship between bond rates and most securities markets). Think about how ridiculous 15% bond rates are. That would mean you could buy what is definitionally the safest investment available and get 15% returns each year. By contrast, the safest stock ETFs (still way riskier than US bonds) average like 7% a year.
So, in the 80s, you could double your money every 5 years while accepting what is usually considered 0 risk. Ever wonder why boomers seemed to get wealthy so easily? This is one of the reasons.
Since the ATH in 1981, the treasury note rate has fallen continuously until Covid hit, at which point it pivoted at a low of about 0.5%. Since then, it has been going back up, but is still extremely low, at around 1.5%.
Since bond rates going down means stocks go up, the stock market has been in a tremendous and arguably unnatural bull run for about 40 years, only pausing twice very briefly for "corrections" circa 1999 and 2008.
Since US bond rates have gotten so close to 0%, they can't really lower them any further without going negative (which is actually a thing, and some countries are trying negative interest now. This is related to Modern Monetary Theory, and is an extremely weird rabbit hole that nobody really knows the true consequences of yet. Imagine getting paid to borrow money. Some countries have been experimenting with this over the last 7 years). The chair of the FED (Jerome Powell) said a few months back that they have no intention of going to negative interest rates, so that means these past 40 years of propping up markets by reducing bond rates has probably come to an end.
You could think of the continuous lowering of rates for the last 40 years as the FED spending its ammunition to prop up markets and propel economic growth, but now that rates are barely above 0% and the FED isn't willing to go negative, they are out of ammo. Not only are they out of ammo when it comes to lowering the rates, but they are also currently incentivized to increase rates to combat rising inflation.
This is why there is fear. The FED has been sticking its hands in for 40 years to prop up markets, and now it seems they are going to stop, at least for now.