The answer is a mix of sensitive 9/11 politics, an unusually powerful bipartisan alliance, election-year timing, and a heavy dose of mistrust and miscommunication between two branches of government that rarely see eye to eye.

The bill itself is not new. For years, victims of the 9/11 attacks and their families have pushed for a change in the law that would exempt acts of terrorism on U.S. soil from the principle of sovereign immunity, which prevents lawsuits against foreign governments and officials in American court. The families want to sue the Saudi government for damages over its alleged ties to the 9/11 hijackers, 15 of whom were Saudi citizens. The Saudis have denied any involvement, and as CIA Director John Brennan made clear on Wednesday, the U.S. government has backed up their denial.

The 9/11 families had two influential senators in their corner: Charles Schumer of New York, likely the next Democratic leader, and John Cornyn of Texas, the second-ranking Republican. With their backing, a revised version of the bill cleared the Judiciary Committee in the spring and then passed the full Senate on a voice vote—a rarity for legislation that drew such vociferous opposition from the White House. Any senator could have objected, but then as now, none wanted to go on record against a proposal billed as “justice for 9/11 families.” Yet because the legislation first passed the Senate without debate, many members only became aware of the administration’s concerns in the last several weeks after Congress returned from a long summer recess. The House passed the bill in similar fashion—without much debate—two weeks ago, and Obama returned it with a veto message last Friday.

“This is a bill that should have been given a greater airing,” Senator Ben Cardin of Maryland told me on Thursday. “It was not on my radar screen until after the train was leaving the station. The next thing I know, it’s on the president’s desk.” Cardin is no backbencher; he is the top Democrat on the Foreign Relations Committee and along with the panel’s chairman, Bob Corker of Tennessee, he spent the last several days scrambling to work out a compromise with the Obama administration that would have kept the bill, as written, from becoming law. Time ran out this week, however, as McConnell decided to bring up the veto-override vote just before lawmakers left to campaign for reelection.

Technically, McConnell could have waited until the lame-duck session in November, allowing more time for debate and the possibility of a compromise. Cardin said, however, that it is Senate custom to act on presidential vetoes as soon as they are received. A Senate Democratic aide, speaking on the condition of anonymity, surmised that the timing had more to do with politics and a pent-up frustration with the outgoing Obama.