More than a few bond bulls out there (those are folks who want rates to go lower!) were eager for the vote to take place on the healthcare bill that drew most of the market's attention this week.  They figured one of two things would happen.  Most likely, there weren't enough votes for the bill to pass.  It would fail, and that would be great for bonds.  

Alternatively, they hoped that if the bill did pass, that it would have been changed so much that it still cast doubt on the new administration's ability to push its policy agenda.  You'd be hard-pressed to find a trader who wouldn't have agreed that one of those 2 outcomes was more likely than a clean passage of the bill.

Instead of any of those options, markets were dealt a wild card in the form of a postponement of the vote.  This worked against bond bulls because it keeps hope alive that politicians can do what politicians do and cobble together some sort of legislation after days and weeks of posturing, wrangling, and horse-trading.  

In other words, the postponement leaves the door open for this whole week to look like normal politics in retrospect.  That's the sort of thing that bond bulls have a healthy fear of, because if it happens with tax reform, it will justify much of the late 2016 rate spike.