I’m thinking two things and they’re related: the first is you didn’t watch the video or couldn’t appreciate it, and the second is that the word flashmob might not have been quite perfect.

One of the key things I think of when I think of a flashmob is the audience having parts join the mob. And also with the flash you think of that happening quickly.

This happened quickly enough but could have happened quicker, but also no audience member became a mob member, they tried and did well to sneak in from behind.

But then again that’s partly due of course to the fact that they’re all carrying medium to large instruments. Hard to blend in in a crowd beforehand.

But they do play a beautiful song very beautifully and many of the crowd’s reactions seem genuine. The band’s reactions seem genuine too, really.

So I think you are misinterpreting a beautiful instance of a song, and that being worth getting choked up over, with a video of something that isn’t quite a flashmob but is titled a flashmob, and from there I presume you either didn’t watch the video but read the title or just somehow did see a flashmob but didn’t see a musical performance.