This happens every time I go to Japan Society in NYC to see an old Japanese film.

I guess it's because it takes effort to get some of those films in good quality or because someone that represents the restoration company is sometimes present, but I don't understand why we're clapping after seeing Yojimbo or some other common film that has been easily available in HD for quite a long time.

Anyway, "Americans clapping after X" has been a meme for some time.