Take market analysis with a grain of salt today.  To a much greater extent than yesterday, market-watchers would have to dig to connect the dots between news/events and market movement.  Volumes were lighter and the shuffling of trading positions looks to have played as big a role as anything else.  

With that out of the way, let's connect the dots as best we can.  Right out of the gate, a headline from an unnamed ECB official (saying a tapering announcement was likely at the September meeting) pushed bond yields noticeably higher.  This occurred just after 7am and resulted in bonds opening weaker for the domestic session.

8:30am economic data wasn't very interesting as far as traders were concerned.  The data also didn't happen to fall very far from forecasts.  The next noticeable move happened during Yellen's Senate testimony.  Apart from a slightly more hawkish tone this time around, Yellen  made a point to go into greater detail on WHY low inflation could be transitory.  

She also said she expects long-term rates to rise as the Fed allows its portfolio to run-off.  While this is a fairly logical conclusion, it prompted a classic "QE-off" move (stock prices lower, bond yields higher) in financial markets.  Stocks recovered in short order, but bonds continued trading near the day's weakest levels, ultimately erasing a small portion of the gains after the 30yr bond auction.