There are numerous biological, physical and neurological factors that can affect cough rate, and implication "it's voluntary" seems to be rather ridi... far-fetched.
Paper link BTW: https://ideas.repec.org/p/cue/wpaper/awp-05-2012.html
I had the same reaction as you though.
My impression is people have respiratory issues, throat irritation, and it's worse during the concert season in dry concert halls, maybe exacerbated by personal fragrances, material from other people in the air and so forth. People try to restrain from coughing but it gets to be too much, and cough during breaks in music because there are no notes to interrupt. Alternatively the coughing is more audible during breaks, so people think it's more common at certain times, but that's only because it's easier to detect the coughing then.
Maybe the paper addresses these things but I can't tell.
I highly doubt it's some form of implicit communication though.
It's easier to hear people cough during a classical concert.
Whether other people are coughing affects whether a person will cough at that moment. That does not mean people cough intentionally.
