Answer:
To contradict the message conveyed by the main plot
A subplot can focus on another aspect of the protagonist, or it can follow the story of another main character and his or her development. It can drive home theme and it can aid in world-building. Subplots will enrich your novel and make your story a more compelling read.
Menelaus wants to live in peace but Odysseus yearns for glory. I believe this is the answer but if not I am super sorry!!
"During the rock concert" would be the subject, as "an amplifier exploded on stage" would be the predicate.
