Sound limiters - the dreaded "clockwork orange". They are usually there because of complaints from neighbours rather than to protect the hearing of those inside the venue. They are a very crude method of controlling sound, and can be set at ridiculously low levels, sometimes less than the volume generated acoustically when they serve no actual purpose other than to disrupt the performance and risk damage to the band's equipment.

I have known them to be triggered by a different band in another room, and by the audience applauding. I probably shouldn't mention the time we ran a long power cable in to the stage from a socket in the kitchen, on a different circuit...

I'm not a fan over over-load music, and now have some hearing loss myself, but these things are just a damned nuisance.