What does the price of crude oil — which has dropped more than 30 percent in the past two months — have to do with this?

American energy companies have been the biggest issuers of high-yield bonds since 2015, according to Dealogic, and their bonds account for about 15 percent of the market. As the price of crude falls, investors’ concerns about the health of these companies and their ability to pay off the bonds grows.

It’s not for nothing. In 2014, when oil prices began to slide, hundreds of smaller drillers went bankrupt, and manufacturers and other businesses that supply the energy industry also took a hit.

[Read more about that risk here.]

But energy companies or high-risk companies aren’t the only reasons for the growing caution. As concerns about the global economy and the strength of earnings mount, investors are demanding more to buy the debt of highly rated companies as well.

Such moves, if they persist, suggest the favorable borrowing conditions that companies have enjoyed for years won’t be as agreeable.