A social urge to show appreciation, even when the intended recipients couldn't ever possibly hear it. I've mostly seen it in cases where excitement was already high, like the first night of a highly-anticipated movie, where something exceptionally amusing just happened.

People sometimes express emotion in ways that don't really make sense.

Or maybe the audience enjoyed the movie?

I don't think it's any different from watching a sporting event and screaming and shouting when the team scores a goal. Even if you're all alone, you want to show appreciation.

Have you ever cried because of a movie? If so, why?

I'd argue crying is a much more primal respons than the somewhat arbitrary slap your open palms together to create a noise response.