Unfamiliar with Concert Customs?

Hello all! I've been listening to the band religiously over the last 6 months and got the chance to attend the concert on Monday in Dallas. The experience was so fantastic and overwhelming that I'm still processing it, and I just remembered an occurrence that left me feeling like a bit of outsider. I can't remember which song it was during, but there was one part where the crowd seemed to yell a certain lyric or phrase incredibly loud. It felt like both Matty and the audience were anticipating it, and it made me wonder what the meaning behind it was and if there exists some fan-related lore around it. Can anyone please fill me in on this? Or was this just a figment of my imagination... Thx!

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