I think we're witnessing right now how reactive BOND is to slight interest rate fluctuations. This ETF has dramatically outperformed the index this past year; it's concerning it could go the other direction, too. Add to this a bull market (for the moment) with lots of sidelined money going that direction and likely cannibalizing the bond side of portfolios.
The advantage of BOND is that it's relatively small so the managers can steer it better than its giant counterparts. The disadvantage, as happened a couple years ago to Gross, is that the managers can steer it the wrong way.
The Morningstar site rates bond funds for rate hike sensitivity: BOND and PTTRX are rated quite sensitive whereas, for example, Doubleline (DBLTX), which I also own, is rated less sensitive.