You don't just pull up to a red light and start clapping. (Wait – right? You don't, do you?) I saw a lady do that today. Just pulled up, alone in her SUV, and started clapping – entirely nonchalantly, as if she was at a show and something just ended. No phone, no companions, no expression on her face – nothing to explain the clapping.
I'm not entirely convinced that "the Pips" isn't some disease that Gladys Knight had at some point early in her career and she named her backup singers that to cover it up when someone overheard her before a show.