There are two times of morning that could be considered "the open" for the bond market.  As far as the futures market is concerned, it's 8:20am as that has long been the opening bell for Treasury futures at the CME.  Since there is no centralized exchange for cash Treasuries, it's really any time that the market decides.  In fact, this is often a moving target some time shortly after 8am.  Recently though, "8am sharp" has seen a surge of trading activity and movement.  Today is one of those days.  It's likely amplified by fact that it's the first trading day of a new month/quarter, which can always bring some random volatility and/or extra momentum in one direction or another.  Today so far, that momentum has been in an unfriendly direction.