Wall Street's biggest bond dealers are finding less and less to like about top-rated U.S. corporate bonds.

So much so that they're betting against some of them, with primary dealers last week turning the most bearish of 2015 on debt maturing in five to 10 years, Federal Reserve data show. The detail on banks' more-granular positions only covers January and February because the Fed just started reporting it.

In any event, it appears the fervent flight to the safety of high-quality corporate debt is coming to an abrupt end. The bonds have declined 1.3 percent in February, poised for their worst monthly decline since the taper tantrum of 2013, according to Bank of America Merrill Lynch index data. That's a reversal of January, when the debt gained 2.7 percent in its best start to a year since 1988.