Let me think....addendum

Other possibilities.

Several people familiar with the diplomacy between the Trump administration and the Taliban puzzled over Mr. Trump’s stated decision to cancel peace negotiations entirely in response to one American casualty, however tragic. The Taliban had not agreed to halt their attacks on Americans in advance of a formal agreement. That raised the question of whether Mr. Trump might have been looking for a pretext because the talks had run into trouble.

  NYT

Oh, I think that's highly likely.

Mr. Trump had been coming under growing pressure from conservatives not to hastily exit the country while many leading Democrats have said they support peace talks leading to an American withdrawal.

Many details of the scrapped Camp David meeting were unclear on Saturday night. The senior Trump administration official said that the decision to cancel the meeting had been made on Thursday, but that Mr. Trump had delayed his announcement. On Friday, Afghan officials confirmed that Mr. Ghani postponed a planned meeting in Washington. One person familiar with the diplomacy said that the plan for a Taliban visit to Washington had not been under discussion until about a week ago. (Taliban representatives have not yet confirmed that they ever planned to attend such a meeting.)

Ha! I guess that WOULD qualify as "running into trouble." Has anyone been keeping track of all the times Trump has announced something that was never settled and had to walk it back or people had to scramble to try to make it happen?

The president has reversed such decisions in short order before. In May 2018, for instance, he abruptly canceled his second summit with Mr. Kim, only to reschedule it days later. But several people familiar with the Afghan talks said on Saturday that it could be difficult to restart them.

[...]

United States and foreign officials said that the talks had reached an advanced stage and, until Saturday night, that an agreement with the Pashtun insurgent group that once harbored the Qaeda mastermind Osama bin Laden was close at hand.

[...]

Mr. Khalilzad had proposed drawing down American military troops in exchange for a partial cease-fire by the Taliban. In a recent interview with the Afghan channel ToloNews, he said 5,400 United States forces would leave Afghanistan within 135 days of a signed agreement.

Under that tentative deal, the number of American troops would have initially been reduced to about what it was when Mr. Trump took office in 2017.

As for the remaining 8,600 American forces, they would have left according to a gradual timeline, perhaps within 16 months.

And that's what he wanted.

“So what comes next in terms of strategic policy options? The two main ones seem to be either keeping the current troop footprint in Afghanistan for the foreseeable future, which Trump clearly doesn’t want to do, or start to draw down anyway, but thus without getting any concessions for it,” said Dan Feldman, who served as special representative for Afghanistan and Pakistan in the Obama administration. “That seems like the worst possible result — withdrawing immediately and irresponsibly, leaving both a security and political vacuum.”

Then that's likely what we'll do.

One Western official said a deal had been nearly at hand but appeared to have been jeopardized by showmanship. Now it has created an environment where the Taliban, as well as a skeptical region that includes Iran and Russia, will conclude that no process with the Americans can be trusted, the official said.

Surely the world is wise to that by now.

I still think I was right about what happened.

[Trump] undboutedly thought that by the shear power of his charisma he would hammer out a peace deal in a couple of hours, announce it on 9/11, and win a Nobel Prize. Then the Taliban blew up an American. Curses! Foiled again!

UPDATE 9/9:  The real reason came out.