By all accounts, it was supposed to be a sleepy August for the U.S. corporate bond market.

Three weeks ago, the thinking went something like this: Sure, the Federal Reserve would cut its benchmark lending rate on July 31, in what Chair Jerome Powell would call a "mid-cycle adjustment." But Treasuries were already pricing in such a move on the short end. Further out on the curve, the 30-year yield was about 2.6 percent, still more than 50 basis points (bps) away from its all-time low. Ten-year yields were about 2 percent, which seemed like a comfortable range for both buyers and sellers. For company finance officers, it had the makings of a sellers' market but one that would be around once summer drew to a close.

Then things got crazy. The 30-year yield lurched lower by 8 bps on August 1, then 13 bps on August 5, then another 13 bps on August 12. After a one-day reprieve near its all-time low of 2.0882 percent, it cruised through that level, tumbling to as low as 1.914 percent. The rally was so intense that the U.S. Treasury Department made an unusual, unscheduled announcement that it was again exploring issuing 50- or 100-year bonds.