America screwed the Taliban govt (which it put into power.. seriously, go check) out of several million US$, gave them none of the aid that they were promised, and abandoned them in a precarious position.
America has, traditionally, treated other countries very poorly.
Eventually, enough people got pissed enough about it, that they decided to do something.
Thought experiment here:
Lets say that you are a disaffected citizen of a country like, say, Afghanistan, or Iraq or somewhere, and that you are very upset with America, and want to make people realise how badly they're screwing over everyone.
What options do you have?
What recourse is open to you?
The only tactic that allows a single individual, or small group, to damage or attack a strong foreign power, is terrorism.