A number of factors appear to be moving markets around on Thursday. In addition to the usual rumors of a "large seller" hitting the sell button, there are at least two developments that are getting a lot of attention:
1) Richard Fisher, the head of the Dallas Federal Reserve, said early Thursday that the Fed may start raising rates in the spring of 2015—earlier than most expected. Of course he is a hawk, but the comments were widely passed around, and he seems to be getting a bit louder as he gets closer to stepping down next April.
In particular, Fisher said "We're beginning to see extreme risk taking in the junk bond markets." Not surprisingly, high yield exchange-traded funds (ETFs) like the iShares High Yield ETF and the SPDR High Yield ETF are down about a half-percent (very large decline for a bond fund) on heavy volume.