This one puzzles me a bit. Is it that the two are playing late at night and ticked off the other people at the motel? The torch,pitchfork,and club reminds me of old Frankenstein movies. Where the villagers attack the castle. Are the two playing the music from that scene and a mob break in thinking there is a monster? Or is it just two nerds playing obnoxious instruments that are getting the rightful wraith that such a monstrous deed requires?
I think it's just that people hate banjo music and hate tuba music so the two of them together translates to the villagers getting restless in Larson's crazy mind.