Treasuries trade almost 24 hours a day (assuming London and Tokyo are not out on holiday), but if the domestic bond market has an unofficial opening bell, it's 8:20am ET. As we often see at the 9:30am NYSE open (or 3pm CME close), buyers or sellers can simply be "lined up." Today, it's sellers. Liquidity has also improved with Treasuries trading in London for the first time this week.
There are no major flashpoints on today's calendar with Pending Home Sales--a report that rarely moves markets--providing the most relevant economic data release. The only other scheduled event with teeth is the 7yr Treasury auction at 1pm ET.