The yield curve is flattening which causes ZB to massively outperform ZN.
Mark Carney, BOE chair flat out said that rate hikes could come earlier than expected, which is rarely said by any central bank chair. This caused a reflexive selloff in shorter maturity bonds, especially the belly of the curve, the 5-7 yr yields.
The Street believes the Long term neutral Fed funds rate will be much lower than before 2008. But they also believe that Fed will raise rates next year, causing short term yields to go up. So the new en vogue trade is the flattener, short the 5 yr and long the 30 yr.
The curve has already flattened tremendously this year, I don't think it can flatten much more unless Yellen hints about faster rate hikes.