Remember when they were in prison and Morty was playing the harmonica and the guy blew up, that was the explanation for the guys blowing up to the harmonica.
The music making them shit thing got foreshaddowed by Morty playing the harmonica in jail and making the inmate shit. So I kept it in mind in expectation of a Mars Attacks! like ending only to get kinda suprised when the gang already figured it out off screen and going full blast xD