So what spurred Trump to threaten a veto in the first place? Here are some likely scenarios, based on how he's handled these negotiations in the past:
1. He was panicking he got shut out of an immigration deal: Trump wanted $25 billion to build his border wall. He got $1.6 billion, and there are limits on what he can physically build with that money.
Democrats were willing to give him most of the wall funding in exchange for protecting hundreds of thousands of young immigrants in the country illegally, known as “dreamers.” But Trump refused to take that deal, saying he wanted limits to legal immigration.
In the end, neither side got anything. And Trump may have missed his last, best chance to get funding for his border wall.
“Trump blew it,” Kevin Appleby, of the Center for Migration Studies, told my colleague Seung Min Kim.
“He feels the media has called him the 'loser' in the negotiations,” said Steve Bell, a former GOP Senate budget aide.
Trump may have been trying to use the only leverage he has left to try to reverse that loss: veto the spending bill unless Congress suddenly agrees to his demands on the border wall and legal immigration. Given that Democrats were willing to shut down the government in January over protecting dreamers, that was a hard sell. So he settled for blasting them in the press conference for "abandoning" dreamers.
2. He didn't mind if a shutdown happened: When you layer on what Trump has said about shutdowns, it is fair to wonder if Trump actually wants one, or at least is okay with one if it happens. My colleagues reported in November that he told confidants a shutdown could be good for him politically, a chance to flex his hard-line muscles on immigration. He's also tweeted stuff like this, saying the country needs a “good” shutdown:
3. He didn't understand what's in the legislation: This wouldn't be a surprise, if it were the case. The president has proven on multiple occasions that he has a nominal grasp, if any, on key issues Congress is debating.
In January, Trump undermined his own White House position when he appeared to oppose Congress's reauthorization of a government surveillance program that allows U.S. intelligence agencies to spy on foreigners. Trump appeared to confuse it with another part of the program, which wasn't up for a vote in Congress that day, that allows the FBI to get a warrant to spy on Americans.
In a bipartisan immigration meeting earlier this year, he almost agreed to approving protections for dreamers without getting any money for his wall. And he may have agreed to that if Rep. Kevin McCarthy (R-Calif.) hadn't stepped in and stopped the president. In the moment, Trump seemed genuinely confused.
In a bipartisan meeting on guns last month, he accused one of the Republicans' most pro-gun control members of being “afraid of the NRA,” which made no sense given Sen. Patrick J. Toomey (R-Pa.) literally wrote the background check bill that nearly passed the Senate in 2013.
This week, House Speaker Paul D. Ryan (R-Wis.) had to head up to the White House during a spring snowstorm to brief the president on what was in the spending bill, after Trump initially balked at some items he misunderstood.
4. He's pulling our attention away from a week of chaos in the White House: This is the ever-present distraction theory that Trump likes to say or do controversial things to distract from other controversial news. It exists because he has been known to do that. Shortly after being inaugurated, Trump accused the Obama administration of wiretapping Trump Tower, without any evidence to back it up. This happened just days after his attorney general recused himself from all Russia matters, an event Trump is still frustrated about.
This week, said budget expert Stan Collender, Trump is facing a lot of potentially damaging news that he may want to bury:
Whatever his reason for suddenly threatening a shutdown, for a few hours it was a real possibility that the government was going to shut down Friday night. The only reason it's not is because Trump changed his mind on the veto threat as abruptly as he issued it.