People are actually wondering why a kick line stirs applause?
Ask yourself, at what point do these mysteriously effective kick lines occur?
Not always, but usually at moments filled with high emotion after a long journey and they give us the sense the characters, dancers, or what have you have finally "arrived" in some better place, situation, time. They punctuate climactic moments so that they are firmly established, that's when the audience intuitively knows that appreciation is warranted.
I guarantee you an audience would sit on their hands if you randomly interrupted a play with a kick line of cheerful looking dancers.
Also, most audiences are looking to have a good time and it's typical to see people become much more attuned to and receptive to emotional, physical, and auditory cues. This is often expressed in an eagerness to "be entertained" and so everything from a bad joke to a mild nuance might incite a much more outwardly response than usual. It can get annoying but it rarely gets to the point of ridiculousness. Like the only performance of Wicked I attended--the girls sitting next to me screeched and howled in rapturous laughter and giggles at every eye wink, lifted eyebrow, gesture that Glinda did. After a while, I dreaded all further comedy since it was met with disruptively eager JOY.
It warms my heart that they had a good time. Really. But come now. Theatre is a communal experience. You're not at home. Be cognizant of that and the rest will flow.
Recreation of original John Cameron orchestration to "On My Own" by yours truly. Click player below to hear.