you don't need to assign motivations to their behavior

Oh but we do; we need to analyse and understand such behaviour if only to effect a cure. I reckon CBT could work wonders. Such people crave vicarious association with the performance owing to deep inner insecurities which obliviates any wider respect for the conventions or the singer. They wish to display equal or superior knowledge through the medium of intimidating mimicry designed to belittle the performer to whom to feel themselves as good, if not better. Much of this is a subconscious dysfunction - possibly genetic (like the Folk Gene itself) and I'd be surprised if any mimic set off to deliberately piss off the singer, just as they're unaware of causing such upset when they do. Even at the Gillian Welch gig in MCR the other night we had to shush someone for singing along; they looked at us as if we'd just deeply insulted them. One woman I told to desist never spoke me again.