> The musicians gradually get tired, and turn up the volume so they don't have to play as hard. Then they get excited and start playing harder. Then they get tired, and turn up the volume.
I think you answered it yourself. Also, from someone who played in a local band for a while (And a not-quite-so-local band thereafter), people who actually know how to use a PA are few and far between. I think most of the time people are lazy and just set the master volume to something that cuts over the chit-chat, then they just don't bother to go back and readjust that when things are quieter/louder.
Seriously folks. If you're performing live, or you bring people in live, make sure at least one of you hires a dedicated, trained PA technician. It's the difference between a mediocre band and a great band (One where you can hear all of the performers), and also helps a lot with ear safety. The performers and the attendees will thank you for it.
At one venue, we just pretended that the house PA was broken.
I'm a musician, and play in wedding bands. I think I've seen enough weddings to suspect that you're probably right about causation.
Something people don't realize about wedding music is that it's not just for dancing. If done well, it can also serve other purposes as well. There will always be a mixed crowd at a wedding, including people who are either unable to fully participate in the socializing for the duration of the party -- e.g., the elderly and children -- or who are shy, or outsiders, whatever. They band can provide them with a focal point and source of entertainment.
At some weddings, the dance floor was taken over by children just tearing around, and at first it would annoy me, but I eventually realized that I was taking pressure off the parents so they could enjoy socializing with one another instead of trying to restrain their kids.
So maybe the real solution is for people to start trying to figure out why the norm for concerts is to have the volume painfully loud. It's possible that guests just might enjoy them more with quieter music.
Self indulgence and ignorance among the performers, including the sound crew if there is one. Everybody believes that their instrument, or their musical style, is the one that needs to be loud. And I freely admit that playing loud is fun.
I've also noticed a sort of ratchet effect. The musicians gradually get tired, and turn up the volume so they don't have to play as hard. Then they get excited and start playing harder. Then they get tired, and turn up the volume. I think that's one explanation for the ever-escalating volume levels.