The joke is the different meaning of the sentence "I wanna rock!" The word "Wanna" is short for both "want a" and "want to" and the word "Rock" is a homonym for a name of a mineral substance, moving something gently (like a baby cradle), or making music. Which is the reason why the person threw a rock at the singer. Not for an act of hatred, but because when the singer said "I wanna rock!", the person must have thought the singer meant "I want a rock" instead of "I want to rock".