They’re actually in the middle of performing Beethoven’s 9th symphony, but there’s a long section near the end where the basses don’t play, so they decide to go to the bar next door and grab a drink.
To know when to come back for the end of the symphony, the bassists tie a string to the conductor’s score a few pages before they start playing again. When he turns the page, it’ll tug on the string and they’ll know to head back to the concert hall.
So the symphony goes on, and pretty soon the conductor realizes he’s in trouble.
It’s the bottom of the ninth, the score is tied, and the basses are loaded.