If stocks close down today, that would mark the third straight day of declines. The declines aren't huge, but in a no-volatility, perpetually rising environment, they stick out.
The big story is the comments from Bank of England chief Mark Carney yesterday night that rate hikes at the BoE could come sooner than markets expect.
It seems very plausible that investors will extrapolate those comments, and apply them to the Fed as well, causing jitters everywhere.