A record portion of the investment-grade bond market is sitting just one step above junk status. If the credit ratings of those companies fall at all, it would greatly increase their likelihood of default. And if a rash of firms default at once, it would put serious pressure on the market, possibly amplifying any imminent recession. Experts at the recent Milken Institute Global Conference broke down their fears around the situation, which are hitting a fever pitch. Visit Business Insider's homepage for more stories.