A specific musical cue--not in the BGM, but in actual music being played by someone or something In-Universe--serves as the trigger for some event. The more dramatic the event, the more likely that the trigger occurs at the end of the song, in which case the song functions as a melodic (and quantum-mechanical-anomaly-proof) form of the Dramatic Countdown Clock. The Xylophone Gag is a very specific subtrope of this. If the song itself is harmful, it's a Brown Note. If the song is part of a magical spell, it's Magic Music. If the song opens a door, it's an example of Songs in the Key of Lock.